feat: throw error when targets are used with regenerator

[agilgur5]

Summary

adds an error when polyfill-regenerator is supplied with targets, as those are unsupported for this polyfill per #36

Details

• the polyfill-regenerator plugin actually does not support targets and
  does not use them

  • it instead detects whether the regeneratorRuntime variable has
    been injected by transform-regenerator, which already supports
    targets
  • so targets are not needed twice in that sense

• instead of silently ignoring the targets option, error out when it is
  given as it is an unsupported option

  • this should hopefully make it easier for developers to realize
    what's going on when debugging issues with this (as I did)

• add a test for this new error as well as a basic "harness" based on
  the one in define-polyfill-provider

  • remove targets from any fixtures that had them for this polyfill
    so that this test passes
    • and because they didn't do anything previously anyway

Review Notes

• I had to create a basic test "structure"/"harness" to check for this error, so let me know if that needs stylistic/structural modification. I tried copying define-polyfill-provider's error testing with a similar options.js test file
• Wording maybe isn't the best, so let me know if you'd like any changes to the error message

References

Fixes #36

[nicolo-ribaudo]

@agilgur5 I was thinking that we could just completely remove the targets options from these plugins: users can specify it as a top-level option.

We would still need a way to pass it as an internal API to the corejs2 and corejs3 plugins (with an "hidden" option similar to "#__secret_key__@babel/runtime__compatibility") for compatibility with @babel/preset-env until Babel 8, but there is no other need to support the targets plugin option now that we have top-level targets.

[nicolo-ribaudo]

As a shorter term solution (that one is a bigger breakig change), we can modify the check like this:

export default defineProvider(({ debug, targets, babel }, options) => {
  if (!shallowEqual(targets, babel.targets())) {
    throw new Error(
      "This plugin does not use the targets option. Only preset-env's targets" +
        " or top-level targets need to be configured for this plugin to work." +
        " See https://github.com/babel/babel-polyfills/issues/36 for more" +
        " details",
    );
  }

where shallowEqual checks if they have the same properties with the same values.

This doesn't catch the case where you explicitly specify the same top-level targets and plugin targets, but it catches the "common error" which is to specify only plugin-level targets (thus not using top-level targets).

[agilgur5]

Rebased & updated with latest changes and added the shallow equality fix for top-level targets. Tests now pass!

[agilgur5]

This is obviously a pretty naive implementation of shallow equality, but it has no deps and is very short. Let me know if that's okay or if you'd prefer something less naive

[nicolo-ribaudo]

I don't like it, but it's ok in this case since it's comparing very small serializable objects.

[agilgur5]

Yep that was kind of my thought process too 👍 Equality algos can get bulky real fast, so kinda went with "smallest viable option" here

[nicolo-ribaudo]

Thanks!