fix: ensure vendored grammar is clean before fetch

[RoloEdits]

The issue came about when trying to do #14814. For some reason the state of the vendored repo was dirty and prevented any fetch:

Fetching 273 grammars
  271 up to date git grammars
  Failure 1/2: markdown_inline Git command failed.
  Stdout:
  Stderr: error: Your local changes to the following files would be overwritten by checkout:
        contrib/screenshot.png
  Please commit your changes or stash them before you switch branches.
  Aborting

  Failure 2/2: markdown Git command failed.
  Stdout:
  Stderr: error: Your local changes to the following files would be overwritten by checkout:
        contrib/screenshot.png
  Please commit your changes or stash them before you switch branches.
  Aborting

Initially I tried to add

git(&grammar_dir, ["reset", "--hard"])?;
git(&grammar_dir, ["clean", "-fdx"])?;

but this only made the CI go from all jobs erroring, to only the tests erroring, with msrv, lint, and doc jobs passing. I also tried to --force on fetch, but the issue remained.

For resolving this, I just went about it the simplest method of just removing the directory before initializing it again, making a clean state to fetch from.

Looking at the diff, github did a pretty horrible job, but basically I reduced nesting in fetch_grammar by utilizing early returns, and encapsulated vendored repo operations with a new VendoredGrammar with various methods that do the same thing as before, now adding a step before fetching to reinitialize the clean repo state.

[RoloEdits]

Forgot to mention, but #14814 is based off of this, and that now seems to no longer have any issues.

[RoloEdits]

CI fail is due to the spurious windows one we have been having for a write test.

[the-mikedavis]

I think we saw this kind of error in the past for one grammar, but it only affected Windows. Removing the directory and checking out from scratch seems like the most straightforward thing to do here 👍