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.