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/RegexMatcher.h

@@ -231,6 +231,7 @@ public:
 private:
     void run_optimization_passes();
     void attempt_rewrite_loops_as_atomic_groups(BasicBlockList const&);
+    bool attempt_rewrite_entire_match_as_substring_search(BasicBlockList const&);
 };
 
 // free standing functions for match, search and has_match