LibRegex: Add the literal string search optimisation

This switches to using a simple string equality check if the regex
pattern is strictly a string literal.
Technically this optimisation can also be made on bounded literal
patterns like /[abc]def/ or /abc|def/ as well, but those are
significantly more complex to implement due to our bytecode-only
approach.

Userland/Libraries/LibRegex/RegexMatch.h

@@ -190,6 +190,11 @@ public:
 
     explicit RegexStringView(DeprecatedString&&) = delete;
 
+    bool is_string_view() const
+    {
+        return m_view.has<StringView>();
+    }
+
     StringView string_view() const
     {
         return m_view.get<StringView>();