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.

helix-term/src/ui/mod.rs

@@ -142,9 +142,6 @@ pub fn regex_prompt(
                                 };
 
                                 cx.jobs.callback(callback);
-                            } else {
-                                // Update
-                                // TODO: mark command line as error
                             }
                         }
                     }

runtime/queries/regex/highlights.scm

@@ -50,4 +50,4 @@
   ])
 
 (class_character) @constant.character
-(pattern_character) @string
+(ERROR) @error