Ignore meta-files from Sapling SCM

[mjmahone]

Sapling is a new incantation of Mercurial, designed to work with git repositories, while supporting Mercurial's preference for stacked commits and stacked rebases as a first-class operation.

This ensures our tooling ignores /.sl directories produced from people who make PRs using Sapling.

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[mjmahone]

This is implemented upstream in prettier prettier/prettier#14206

So I'll close this as, when we upgrade prettier after the next prettier release, we'll get this functionality for free!