Tune regex highlights for usage in prompts

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.

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