Preserve separators in numeric literals with target=ES2021+

[danvk]

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Fixes #53182

The issue was that back when numeric separators were first implemented in TypeScript back in 2017 (#20324 (review)) there was no ES2021 emit target (or ES2018 for that matter!) and so ESNext was used as a stand-in. When target=ES2021 was added later, the check for numeric separators wasn't updated from ESNext → ES2021.

[danvk]

@microsoft-github-policy-service agree

[sandersn]

This looks OK to me. @DanielRosenwasser @rbuckton is this safe to put in 5.4? Do we want it there? It's technically a fix but @RyanCavanaugh treated it as a bonus feature in the bug.

[RyanCavanaugh]

Not a feature
Not a bug
Some secret third thing?

[RyanCavanaugh]

TODO: Figure out how to avoid this kind of bug in the future? It's come up multiple times that people compare against ESNext because we don't know which ES20XX version the spec update will land in, then we forget to update the code. Maybe we should make ES2099 as an internal-only name and then rename it to e.g. ES2026 when the spec versions land?

[sandersn]

I think the right answer is a manual sweep of esnext each year as the standard is finalised. From asking @rbuckton yesterday we learned that it's usually April (provisional) or June (final).

There's no way to predict with ES20xx a particular addition will end up in, so I don't think renaming a placeholder will help. Even if each proposal gets its own placeholder, they'd still need a manual update to the year that they actually ended up in.

[danvk]

Let's all hope we're around to see the final feature set of ES2099! 😀

[RyanCavanaugh]

Sowing the grounds for the first Y2.1K bug 😭