ESLint plugin for moonlight pitfalls

[NotNite]

We tell people to not fall for pitfalls but it's hard. Cyn just wasted an unknown number of hours because tsc autocompleted a Flux store import and created a duplicate copy of it. We should look into how ESLint plugins work and use one in here & in the sample extension.

Some ideas:

• directive to mark a file as never to be imported directly (this one is easy)
• enforcing the moonlight env globals to be used only in the right entrypoints (probably impossible)
• checks for using an extension or mapped module without it marked as a dependency (would require lots of AST bullshit)
• checks for using Spacepack in a Webpack module without the needs declared (probably very hard if not impossible)