This PR adds support for detecting projects using Sapling VCS.

Sapling is a Meta maintained fork of Mercurial. It uses .sl as the state directory instead of .hg to avoid compatibility issues with Mercurial. See this page for comparison with Mercurial: https://sapling-scm.com/docs/introduction/differences-hg

Testing

1. Load projectile with changes from this PR
2. Open a project managed with Sapling
3. Run M-x projectile-project-info:

4. Run M-x projectile-find-file:

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