Add support for a `padded` directive

[lmanini]

Overview

A PR to support the request from issue #301.

Made minimal changes to add support for a padded keyword, adding it to the langauge grammar and allow for a macro to have a #define padded directive within it. The parser's Statement generation LGTM upon a manual inspection of the (failing) added test in huff_parser/tests/padded.rs, although I'm currently stuck, as I don't how to handle the generated stop opcodes ast Spans, given they're not present in the source file. What should we set their Span to?

Looking for suggestions on how to solve this issue and feedback on the work done sor far!