Split out JSX-related rules into separate package

Currently this plugin pulls a big load of jsx-related dependencies to support the jsx rules, but these are useless dependencies for users who do not use jsx.

How about splitting those rules out into separate package `eslint-plugin-github-jsx`?

The issue names no files or tests. Start by inventorying the JSX rules and their dependencies in eslint-plugin-github, then define the package boundary; done means JSX users can use eslint-plugin-github-jsx while users without JSX no longer install those dependencies.

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