Clarify grammar workflow and move grammar to this repo

[albertjan]

I vote 3...
so then the workflow for working with the grammar can be something like:

Generate patch

copy python.gram into skulpt_parser somewhere (maybe tools/grammar_patch)
git diff with skulpt.dev.gram and copy the output to the same folder
Then either as the apply_grammar_patch or just as step 3 in the above.
apply the patch to skulpt.gram somewhere (maybe src/grammar)

Then I think we can abandon #12 and it can just live in the tools/grammar_patch folder as a future helper/reference function.
(I'll adjust that pr after this pr)