[CHORE] Use maintained fork of `gray-matter`

Currently packages/cli/package.json and packages/devtools-server/package.json point to gray-matter v4.0.3.

That gray-matter repo appears to be unmaintained with it's last release was in January 2019: https://github.com/jonschlinkert/gray-matter/tags

Luckily 11ty maintain a fork of this library: https://github.com/11ty/gray-matter which means that its dependencies are also upgraded as there are CVEs etc.

I propose that refine should move to this maintained version of the gray-matter library. I'll put in a PR with the required changes.

This was provoked by a recent security alert on js-yaml that is a dependency of both gray-matter and @11th/gray-matter but is only likely to be addressed by @11ty/gray-matter.