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.cpp

@@ -419,6 +419,26 @@ private:
 template<class Parser>
 bool Matcher<Parser>::execute(MatchInput const& input, MatchState& state, size_t& operations) const
 {
+    if (m_pattern->parser_result.optimization_data.pure_substring_search.has_value() && input.view.is_string_view()) {
+        // Yay, we can do a simple substring search!
+        auto& needle = m_pattern->parser_result.optimization_data.pure_substring_search.value();
+        if (needle.length() + state.string_position > input.view.length())
+            return false;
+
+        auto haystack = input.view.string_view().substring_view(state.string_position);
+        if (input.regex_options.has_flag_set(AllFlags::Insensitive)) {
+            if (!haystack.substring_view(0, needle.length()).equals_ignoring_ascii_case(needle))
+                return false;
+        } else {
+            if (!haystack.starts_with(needle))
+                return false;
+        }
+
+        state.string_position += needle.length();
+        state.string_position_in_code_units += needle.length();
+        return true;
+    }
+
     BumpAllocatedLinkedList<MatchState> states_to_try_next;
 #if REGEX_DEBUG
     size_t recursion_level = 0;