feat: ESLint Language Plugins (#99)

* feat: ESLint Language Plugins

* More details

* More details

* Notes on JS changes

* Finished the RFC

* Mention alternate proposal

* Add RFC link

* Update designs/2022-languages/README.md

Co-authored-by: Brandon Mills <btmills@users.noreply.github.com>

* Update designs/2022-languages/README.md

Co-authored-by: Brandon Mills <btmills@users.noreply.github.com>

* Incorporate first bits of feedback

* Flesh out SourceCode interface

* Add example for getVirtualFiles()

* Remove getVirtualFiles()

* Revert language options schema

* esquery changes

* SourceCode changes

* Simplify violation interface

* Added FAQs about rules and autofixing

* FAQ update

* Add matchesSelectorClass() method

* Update designs/2022-languages/README.md

Co-authored-by: James Henry <james@henry.sc>

* Update ParseResult definition

* Clean up errors and violations types

* Update method signatures

* Make ParseResult#errors optional

* Update based on esquery PRs

* Add visitorKeys

* Finish sentence

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Co-authored-by: Brandon Mills <btmills@users.noreply.github.com>
Co-authored-by: James Henry <james@henry.sc>