Add es-check as CI step in Next pipeline

[merrywhether]

Describe the feature you'd like to request

Next currently officially supports ES5 builds for client code: #33227

There have been a few recent bugs related to shipping non-ES5 code in releases:

• Vendored useSubscription is not ES5-compatible #35912
• Image config from server (not transpiled) being imported in client #34629
• Client bundles contain non es5 code #33314

As a consumer of Next since v5, it seems like the frequency of these regressions has been increasing recently, which is understandable as modern JS moves further and further from ES5. Next should have a CI step that prevents the introduction of these regressions as long as ES5 is the official target. This would prevent consumers from getting blocked on upgrades and having to report these bugs and then waiting for the next release (which can be understandably slow).

Describe the solution you'd like

es-check is a lightweight fast utility that is capable of preventing these types of regressions. Next should introduce its usage as a CI step. The easiest path forward would be running it against the built static output in pre-existing steps (like the e2e or production test suites maybe?). Maybe use the existing open #35912 ticket as a way to test efficacy, since its codepath (next/dynamic) isn't necessarily invoked in many setups.

Describe alternatives you've considered

es-check is the defacto tool in this situation, as evidenced by all of the bug reports mentioning it. It's possible that this could be caught in some sort of browser grid testing, but that would be much slower and might not address all browsers Next consumers are targeting while relying on ES5.