With this patch, the blocks in a markdown document render a vector of
lines. These lines get concatenated in Document::render_to_terminal, so
this does not change any external APIs of LibMarkdown.
This change makes it possible to indent individual lines in the rendered
markdown. So, rendering blockquotes in a similar way to code blocks :^)
We have a new, improved string type coming up in AK (OOM aware, no null
state), and while it's going to use UTF-8, the name UTF8String is a
mouthful - so let's free up the String name by renaming the existing
class.
Making the old one have an annoying name will hopefully also help with
quick adoption :^)
This also improves Commonmark coverage, e.g. it fixes tests
HTML_blocks_ex179_2894..2906 and Lists_ex308_5439..5457.
In other words, we go from 271 out of 652 to 273 out of 652.
