feat: conditional exports

[ricokahler]

This PR:

1. Adds conditional exports to color2k (see below)
2. Includes allow custom background color #405
3. Cleans up some ESLint package deps

Color2k handles its own bundle with the following entry points and targets:

• main — node 4
• unpkg — browserslist defaults
• module — node 6 for ES2015 support defined by rollup a while ago
• exports.import — node 12 for ES2019 support (esbuild and node >12 in ES mode uses this entry point)
• exports.default — node 12 for CJS node 12 support, since export maps weren't support until node 12
• exports.package.json — for backwards compat to importing package.json (since it's restricted to import other files now)

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The following consumers have been manually tested:

• legacy node (e.g. node 4)
• <script type="module"> via skypack
• deno import via skypack
• jest with the default babel setup — this uses jest-resolve which uses resolve (maintained by none other than @ljharb! 😄). This works but uses the legacy main CJS bundle due to the complexity of closing Support ESM resolution browserify/resolve#222. Note from the jest folks, they are highly unlikely to switch from resolve.
• @babel/register with [@babel/preset-env, {targets: 'node 12'}] — uses the CJS exports.require entry point
• @rollup/plugin-node-resolve — uses the exports.import entry point
• webpack 5 — uses enhanced-resolve with the conditions ["require", "module"] for CJS and ["import", "module"] for ESM. resolves to the exports.import entry point
• webpack 3 — this works and uses the legacy module entry point
• node LTS with both ESM/CJS — works as expected

Closes #413

[codecov]

Codecov Report

Merging #419 (d1c87a7) into main (37af5b7) will increase coverage by 0.01%.
The diff coverage is 66.66%.

@@            Coverage Diff             @@
##             main     #419      +/-   ##
==========================================
+ Coverage   96.44%   96.46%   +0.01%     
==========================================
  Files          23       23              
  Lines         197      198       +1     
  Branches       44       45       +1     
==========================================
+ Hits          190      191       +1     
  Misses          7        7              

Impacted Files	Coverage Δ
src/hasBadContrast.ts	80.00% <66.66%> (+5.00%)	⬆️

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[ljharb]

This works in theory :-) i'd have to look at the build output to be sure.

[ljharb]

seems good; short of an actual node 4 test

[ricokahler]

@ljharb alright i've spent waaaay to long reading up on these exports and manually testing but I think we're finally good to go after a node 4 test. I'll merge after getting that done.

Thanks for your feedback!

[ljharb]

I will point out that adding "exports" is almost always a breaking change, so you may want to do this as a semver-major bump.

[github-actions]

🎉 This PR is included in version 2.0.0 🎉

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

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