feat: Snakemake grammar and parser

Originally posted on discord by @mbhall88

[...] there are a number of outstanding bugs for snakefmt which Brice and I are struggling to keep on top of as we don't have a lot of bandwidth for this repo. The main challenge is that fixing a bug requires quite a lot of walking through with the debugger very carefully which takes considerable time. I think ultimately all of these issues could be made much easier if there were a universal snakemake parser. Maybe something like a Lark parser (https://github.com/lark-parser/lark/). However, this would require a snakemake grammar, which does not seem to exist at the moment. This has come up in a number of github issues https://github.com/snakemake/snakemake/issues/2224 https://github.com/snakemake/snakemake/issues/1502 https://github.com/snakemake/snakemake/issues/1168 I think a grammar is a vital and necessary part of having a DSL like snakemake. It would make our lives significantly easier in snakefmt and would mean that updates to the snakemake language wouldn't require code changes in downstream tools that need to parse snakemake files. Is this something you and your team could/will work on?