Update: add es2021 environment (refs #13602)

[mdjermanovic]

Prerequisites checklist

• I have read the contributing guidelines.

What is the purpose of this pull request? (put an "X" next to an item)

[X] Add something to the core

refs #13602

What changes did you make? (Give an overview)

• Added new es2021 environment with FinalizationRegistry and WeakRef globals. edit: this is in @eslint/eslintrc now.
• Updated --init to enable the new environment in created configs.
• Updated configuring.md with the new environment.
• Also updated configuring.md and types.js with ecmaVersion: 2021. edit: done in Upgrade: espree@7.3.0 (refs #13568) #13609
• Also updated some tests where it seemed the intention was to use the latest es environment and/or the latest ecmaVersion. If that wasn't the case, I can revert those changes.

Is there anything you'd like reviewers to focus on?

• package.json has github espree reference.
• Noticed while rebasing at the end that this PR changes some files @nzakas just marked as "do not modify".

[nzakas]

This change is okay; I can re-sync to pull these into my work. We just can’t keep making changes on config-related files because I’ll never be able to keep up.

[mdjermanovic]

Rebased to update package.json and undo some changes that were also made in #13609. Also added one no-undef test that might be useful since we don't have specific tests for environments.

[mdjermanovic]

Rebased again to fix merge conflicts. In particular, to undo changes in conf/environments.js since that file isn't in this package anymore.

[kaicataldo]

LGTM, thanks!