Validate `editor.formatOnType` validity using Python's test grammar

[brettcannon]

https://github.com/python/cpython/blob/master/Lib/test/test_grammar.py

We should run this file through tests at least once to make sure things work, but it's probably better to write a test for this as well to prevent regressions (we will need to make sure to update it for each new release of Python).

[brettcannon]

And if we do embed the file as a test then we will need to add the Python license appropriately.

[brettcannon]

I found another bug in the editor.formatOnType support, so doing this validation is getting important.