We can use tree-sitter-regex highlighting in prompts for entering
regexes, like `search` or `global_search`. The `highlighted_code_block`
function from the markdown component makes this a very small change.

This could be improved in the future by leaving the parsed syntax tree
on the prompt, allowing incremental updates. Prompt lines are usually so
short though and tree-sitter-regex is rather small and uncomplicated,
so that improvement probably wouldn't make a big difference.

Since regex is almost always injected into other languages,
`pattern_character`s will inherit the highlight for the structure that
injects them (for example `/foo/` in JavaScript or `~r/foo/` in Elixir).
This removes the string highlight when used in the prompt.

We also add `ERROR` node highlighting so that errors in regex syntax
appear in the prompt. This resolves a TODO in the `regex_prompt`
function about highlighting errors in the regex.

Pascal and I discussed this and we think it's generally better to
take a 'RopeSlice' rather than a '&Rope'. The code block rendering
function in the markdown component module is a good example for how
this can be useful: we can remove an allocation of a rope and instead
directly turn a '&str' into a 'RopeSlice' which is very cheap.

A change to prefer 'RopeSlice' to '&Rope' whenever the rope isn't
modified would be nice, but it would be a very large diff (around 500+
500-). Starting off with just the syntax functions seems like a nice
middle-ground, and we can remove a Rope allocation because of it.

Co-authored-by: Pascal Kuthe <pascal.kuthe@semimod.de>

This removes a handful of allocations for functions calling into the
function, which is nice because the prompt may call this function on
every keypress.