AK: Add StringView::find_last_not

Author: https://github.com/cqundefine
Commit: 9718667bcf
Pull-request: https://github.com/SerenityOS/serenity/pull/15278
Reviewed-by: https://github.com/ADKaster
Reviewed-by: https://github.com/Quaker762
Reviewed-by: https://github.com/bgianfo
Reviewed-by: https://github.com/timschumi

AK/StringUtils.h

@@ -74,6 +74,7 @@ StringView trim_whitespace(StringView string, TrimMode mode);
 Optional<size_t> find(StringView haystack, char needle, size_t start = 0);
 Optional<size_t> find(StringView haystack, StringView needle, size_t start = 0);
 Optional<size_t> find_last(StringView haystack, char needle);
+Optional<size_t> find_last_not(StringView haystack, char needle);
 Vector<size_t> find_all(StringView haystack, StringView needle);
 enum class SearchDirection {
     Forward,