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/RegexParser.h

@@ -55,6 +55,10 @@ public:
         Token error_token;
         Vector<DeprecatedFlyString> capture_groups;
         AllOptions options;
+
+        struct {
+            Optional<DeprecatedString> pure_substring_search;
+        } optimization_data {};
     };
 
     explicit Parser(Lexer& lexer)