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/DevTools/HackStudio/LanguageServers/Cpp/CppComprehensionEngine.cpp

@@ -359,7 +359,7 @@ String CppComprehensionEngine::document_path_from_include_path(const StringView&
         if (!library_include.search(include_path, result))
             return {};
 
-        auto path = result.capture_group_matches.at(0).at(0).view.u8view();
+        auto path = result.capture_group_matches.at(0).at(0).view.string_view();
         return String::formatted("/usr/include/{}", path);
     };
 
@@ -368,7 +368,7 @@ String CppComprehensionEngine::document_path_from_include_path(const StringView&
         if (!user_defined_include.search(include_path, result))
             return {};
 
-        return result.capture_group_matches.at(0).at(0).view.u8view();
+        return result.capture_group_matches.at(0).at(0).view.string_view();
     };
 
     auto result = document_path_for_library_include(include_path);