A TextDocumentLine is now backed by a non-null-terminated sequence of
Unicode codepoints encoded as UTF-32 (one u32 per codepoint.)
This makes it possible to view and edit arbitrary Unicode text without
strange cursor and selection behavior. You can freely copy and paste
emojis between TextEditor and Terminal now. :^)
Storing UTF-32 is quite space-inefficient, but we should be able to
use the same optimization techniques as LibVT does to reduce it in
the typical case where most text is ASCII.
There are a lot of things that can be cleaned up around this code,
but this works well enough that I'm pretty happy with it.
