LibRegex+Everywhere: Make LibRegex more unicode-aware

This commit makes LibRegex (mostly) capable of operating on any of
the three main string views:
- StringView for raw strings
- Utf8View for utf-8 encoded strings
- Utf32View for raw unicode strings

As a result, regexps with unicode strings should be able to properly
handle utf-8 and not stop in the middle of a code point.
A future commit will update LibJS to use the correct type of string
depending on the flags.

Userland/Utilities/expr.cpp

@@ -417,7 +417,7 @@ private:
 
                 StringBuilder result;
                 for (auto& e : match.capture_group_matches[0])
-                    result.append(e.view.u8view());
+                    result.append(e.view.string_view());
 
                 return result.build();
             }