Mozlint provides two main benefits:
1. A common system for defining lints across multiple languages
2. A common interface and result format for running them
This commit only adds the core library, it does not add any consumers of mozlint just yet.
MozReview-Commit-ID: CSQzq5del5k
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extra : rebase_source : b520b96177281a1b1770edf53a01cbc2196f494f
