Add Conditional Exports in Popper v2

[chadrschroeder]

Fixes #1465.

This pull request is similar to #1502, but for the v2.x branch.

I've left all existing distributions in place and have added 2 new files to use in the development and production conditional exports:

Exports Entry	File Name	Notes
import.development	dist/esm/index.development.js	New un-minified ESM build with debugging on.
import.production	dist/esm/index.min.js	New minified ESM build with debugging off.
import.default	lib/index.js	Existing ESM build. Same as module.
require	dist/cjs/popper.js	Existing CJS build. Same as main.

I copied the entire "exports" format from the current version which means I also brought back the update from #1462. I can leave that out if you'd prefer.

Please let me know if I'm doing anything wrong here or if there's a different approach you would recommend.

[rollingversions]

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[chadrschroeder]

Because I didn't want to alter anything about the existing build, __DEV__ will still be replaced by babel-plugin-dev-expression. Then I replace the process.env.NODE_ENV reference after that.

[dominikg]

this was added before #1342 and rolled back #1362 due to it being a breaking change. (adding an exports map always is)

I think it is worth supporting esm with export maps but it would have to be in a new major release.

[atomiks]

Based on Rich's comment here: sveltejs/kit#4504 (comment)

Would it be enough to create a new mjs folder build? esm used to have .mjs as the extension name, but it changed to .js in v2.0.4 for some reason — at this point, it's probably too major of a breaking change to revert that back to .mjs, right? I don't think we can add an export map at all though.

But we could add a new folder using that extension. Libraries could then import from that folder instead of /esm, which would basically be the same as a new major.

[chadrschroeder]

I'm going to close this PR. I'm OK with sticking with how V2 works at this point. If someone else has a different need that the new mjs folder would help out with, I'll let them go for it in another PR.

Thank you for working on the export maps in the current version of floating-ui! I'm looking forward to when I'll be able to use that.