Support WeakRefs

[mdjermanovic]

WeakRefs proposal is now Stage 4.

https://github.com/tc39/proposal-weakrefs

(it seems the proposal's repo hasn't been updated yet with tc39/proposal-weakrefs#212, but it was official in tc39/proposals@489f6df and merged in the spec in tc39/ecma262@71c0897).

• ESLint Update 1 (minor version)
  • new es2021 environment with FinalizationRegistry and WeakRef globals. Update: add es2021 environment (refs #13602) #13603
• ESLint Update 2 (major minor version):
  • no-extend-native Update: report es2021 globals in no-extend-native (refs #13602) #14177 Edit: per the latest update in our semver policy, we can release this change in a minor version.

Not sure if the new syntax label is appropriate since this isn't "syntax".

[mdjermanovic]

As for no-extend-native, this rule currently doesn't report any errors in the following code:

/* global FinalizationRegistry, WeakRef */

FinalizationRegistry.prototype.foo = bar;
WeakRef.prototype.foo = bar;

It will start reporting this code, but not before the next ESLint major version when we will upgrade the globals package to ^13 or above.

The same applies to a configuration where the new es2021 environment (which contains FinalizationRegistry and WeakRef) is enabled.

So, regardless of user configuration, the no-extend-native rule will not disallow modifying FinalizationRegistry and WeakRef prototypes until ESLint v8.0.0.

This is in line with a similar decision in #11803 for new BigInt globals, where it was seen as a breaking change, and thus postponed for v7.0.0 (though, for most users the change probably happened before v7.0.0, because globals added BigInt globals in a minor version of the package, and we have ^ dependency).

[mdjermanovic]

Marked as breaking for Update 2: upgrading globals package, which would cause no-extend-native to report more errors.

[mdjermanovic]

Per our updated semantic versioning policy, we merged the change in no-extend-native rule for a minor release, so this is now all done.