This will make it easier to support both string types at the same time
while we convert code, and tracking down remaining uses.
One big exception is Value::to_string() in LibJS, where the name is
dictated by the ToString AO.
This moves all code comprehension-related code to a new library,
LibCodeComprehension.
This also moves some types related to code comprehension tasks (such as
autocomplete, find declaration) out of LibGUI and into
LibCodeComprehension.
This adds a new semantic token type, PreprocessorMacro.
Calls to preprocessor macros will now be highlighted when semantic
highlighting is enabled in Hack Studio.
The SemanticSyntaxHighlighter uses TokenInfo results from the
language server to provide 'semantic' syntax highlighting, which
provides more fin-grained text spans results.
For example, the SemanticSyntaxHighlighter can color function calls,
member fields references and user-defined types in different colors.
With the simple lexer-only syntax highlighter, all of these tokens were
given the same text highlighting span type.
Since we have to provide immediate highlighting feedback to the user
after each edit and before we get the result for the language server,
we use a heuristic which computes the diff between the current tokens
and the last known tokens with compete semantic information
(We use LibDiff for this).
This heuristic is not very performant, and starts feeling sluggish with
bigger (~200 LOC) files.
A possible future improvement would be only computing the diff for
tokens in text ranges that have changes since the last commit.
