Teach GitHub how to recognize ytt YAML as a language

https://github.com/github/linguist/blob/master/CONTRIBUTING.md#adding-a-language

Overall:
- [ ] find 200+ unique GitHub repos that are using ytt

For ytt modules (templates, overlays, library files) — `.yml`, `.yaml`:
- [ ] write a syntax grammar
- [ ] get the Bayesian classifier to correctly pick out ytt from a file of `.yml`/`.yaml` files of various dialects
- [ ] create PR with required "evidence" that this language is in wide use and can be successfully identified most of the time.

For ytt-flavored Starlark,
- [ ] ?

Start with CONTRIBUTING.md's “Adding a language” section and the linked GitHub Linguist guidance. Establish the required evidence from 200+ repositories, then scope the grammar and classifier work for .yml/.yaml and resolve the ytt-flavored Starlark question; done means the contribution meets Linguist's evidence and identification requirements.

Written by the indexing model from the issue text.