AK: Add String{,Utils}::to_snakecase()

This is an improved version of WrapperGenerator's snake_name(), which
seems like the kind of thing that could be useful elsewhere but would
end up getting duplicated - so let's add this to AK::String instead,
like to_{lowercase,uppercase}().

AK/StringUtils.h

@@ -72,6 +72,8 @@ bool contains(const StringView&, const StringView&, CaseSensitivity);
 bool is_whitespace(const StringView&);
 StringView trim_whitespace(const StringView&, TrimMode mode);
 Optional<size_t> find(const StringView& haystack, const StringView& needle);
+String to_snakecase(const StringView&);
+
 }
 
 }