Integrating SWEET parser (#265)

* feat: sweet parser scaffolding

* refactor: remove redundant code, offloading work to parser crate

* feat: move modifier validation to parser

* feat: use readily available mode instructions from parser

* deps: bump sweet version

* chore: remove unused grammar constants

* docs: update docs for `sweet_def_to_kb` function

* tests: finished porting all tests

* fix: there are no swhkd tests

* fix: change sweet source url to in-tree repo

* chore: `sweet` use re-exported modules from crate root

Author: lavafroth