feat(plugin-pack): ignore Sapling from `yarn pack`

[vegerot]

Summary: This change adds .sl to the list of directories ignored by yarn pack.

Test Plan:

• Clone a Sapling repo
• Put some JS files in the .sl directory (e.g. touch .sl/package.json)
• Run yarn pack
• Verify that the .sl directory is not included in the tarball

[vegerot]

question for reviewer: did I do the version update correctly? I think plugin-pack should be a minor release because it's adding a feature, but idk what the other plugins that depend on plugin-pack should be.