Documentation: Add Neovim configuration documentation

Documentation/NvimConfiguration.md

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+# NVim Project Configuration
+
+NVim can be configured to use the [COC-clangd](https://github.com/clangd/coc-clangd)
+plugin to provide code-completion as well as inline
+[git blame](https://github.com/f-person/git-blame.nvim) using [vim-plug](https://github.com/junegunn/vim-plug).
+
+Make sure you ran `./Meta/serenity.sh rebuild-toolchain` as well as
+`./Meta/serenity.sh run`.
+
+# Install vim-plug
+
+See [https://github.com/junegunn/vim-plug](https://github.com/junegunn/vim-plug).
+
+# Install coc.nvim
+
+The config file for neovim is at `~/.config/nvim/init.vim` or if
+set `$XDG_CONFIG_HOME/nvim/init.vim`.
+
+Add the Plugin:
+
+```vim
+Plug 'neoclide/coc.nvim', { 'branch': 'release' }
+```
+
+Run `:PlugInstall` inside nvim.
+
+# Install coc-clangd via CocInstall
+
+```vim
+:CocInstall coc-clangd
+```
+
+> **Note**: This guide is tested with clangd version 14.0.6 and 15.0.6.
+
+In case you have not installed clangd already install it with
+`:CocCommand clangd.install`.
+
+This will install a separate version of clangd just for neovim.
+
+# Configure coc-clangd in coc-settings.json
+
+Use the following settings to ensure that coc-clangd works out of the box.
+
+> **Note**: You might want to adjust the `clangd.fallbackFlags` depending on your build
+system and customize the `inlayHints.sep` based on your preference.
+
+```json
+{
+    "clangd.fallbackFlags": ["-std=c++20"],
+    "semanticTokens.enable": true,
+    "inlayHint.subSeparator": "︴",
+    "inlayHints.enableParameter": true,
+    "clangd.inlayHints.sep": "⇝"
+}
+```
+
+To change the coc-settings.json go to the file `~/.config/nvim/coc-settings.json`
+or type `:CocConfig` in the command line.
+
+> **Note**: In case you already had another c++ language server configured in the
+`coc-settings.json` you might want to nuke it first and
+work towards your desired config by adding the other parts back in to avoid
+conflicts.
+
+> **Note**: If you have configured `clangd` as a languageServer in
+`coc-settings.json`, you should remove it to avoid running clangd twice!
+
+> **Note**: `clangd.inlayHints.sep` breaks on `clangd 15.0.6`.
+
+# Install git blame (Optional)
+
+```vim
+Plug 'f-person/git-blame.nvim'
+```
+
+Run `:PlugInstall` inside nvim.
+
+# Configure your init.vim
+
+The config file for neovim is at `~/.config/nvim/init.vim` or if
+set `$XDG_CONFIG_HOME/nvim/init.vim`.
+
+The `init.vim` excerpt:
+
+```vim
+"IMPORTANT: the leader key for <leader> keycombos
+let mapleader = "\\"
+
+"BEGIN: git blame (optional)
+hi GitBlame guifg=#7b7b7b
+let g:gitblame_date_format = '%d.%m.%y %H:%M'
+let g:gitblame_highlight_group = 'GitBlame'
+let g:gitblame_message_when_not_committed = 'You: Uncommitted changes'
+let g:gitblame_message_template = '   <author> (<committer>), <date> <sha> • <summary>'
+"END: git blame
+
+"BEGIN: coc
+"inline hints (depending on clangd version one or another gets used)
+hi CocHintVirtualText guifg=#84afe0
+hi CocInlayHint guifg=#84afe0 guibg=#393939
+hi CocInlayHintParameter guifg=#84afe0 guibg=#393939
+hi CocInlayHintType guifg=#89ddff guibg=#393939
+
+"semantic highlighting
+hi CocSemMethod guifg=#bfaa87 gui=bold
+hi CocSemFunction guifg=#bfaaf7 gui=bold
+hi CocSemParameter guifg=#a9bfd1 gui=underline
+hi CocSemVariable guifg=#8edbdb
+hi CocSemProperty guifg=#23ce6d
+hi link CocSemEnumMember Constant
+hi link CocSemEnum CocSemClass
+hi Constant guifg=#f78c6c
+hi CocSemClass guifg=#89ddff
+hi Statement guifg=#c792ea
+hi Type guifg=#db954a
+
+
+"remap keys for applying refactor code actions (on warnings) (\re)
+nmap <silent> <leader>re <Plug>(coc-codeaction-refactor)
+xmap <silent> <leader>r  <Plug>(coc-codeaction-refactor-selected)
+nmap <silent> <leader>r  <Plug>(coc-codeaction-refactor-selected)
+
+"outline for file (\o)
+nmap <silent><nowait> <leader>o :<C-u>CocList outline<cr>
+
+"goto definition etc.
+nmap <silent> gd <Plug>(coc-definition)
+nmap <silent> gt <Plug>(coc-type-definition)
+nmap <silent> gi <Plug>(coc-implementation)
+nmap <silent> gr <Plug>(coc-references)
+
+"coc rename (\rn)
+nmap <leader>rn <Plug>(coc-rename)
+
+"prev or next error
+nmap <silent> [g <Plug>(coc-diagnostic-prev)
+nmap <silent> ]g <Plug>(coc-diagnostic-next)
+
+"confirm coc-suggestion with enter
+imap <silent><expr> <CR> coc#pum#visible() ? coc#pum#confirm() : "\<CR>"
+
+"ctrl+space for completion
+imap <silent><expr> <c-space> coc#refresh()
+
+"show documentation with ctrl+k
+nmap <silent><c-k> :call ShowDocumentation()<CR>
+
+"show documentation if it's available
+function! ShowDocumentation()
+  if CocAction('hasProvider', 'hover')
+    call CocActionAsync('doHover')
+  else
+    call feedkeys('K', 'in')
+  endif
+endfunction
+
+"coc-clangd switch between header and source
+nmap <silent>gs :CocCommand clangd.switchSourceHeader vsplit<CR>
+"END: coc
+```
+
+# Configure .clangd
+
+> **Note**: Every time a new source is added or the compilation commands get adjusted
+(through CMake) you need to rerun `./Meta/serenity.sh rebuild`.
+
+Link `ln -s /path/to/serenity/Build/x86_64/compile_commands.json /path/to/serenity/compile_commands.json`.
+
+Create `/path/to/serenity/.clangd` (replace `/path/to/serenity`
+with your SerenityOS directory) with content of the clangd section in the
+[VSCodeConfiguration.md](./VSCodeConfiguration.md).
+
+> **Note**: You can add a `Remove` part, where you can remove unwanted flags
+such as those that aren't supported by the current version of clang.