Investigate strengthening checks that transpiled code matches browser support

[romaricpascal]

What

See if we can improve our confidence that the code we ship, which is transpiled by Babel, matches our browser support (ie. doesn't require any surprise polyfill or leftover syntax that wouldn't run everywhere).

There are various times this could happen:

• during review of PRs enabling new syntaxes (review of browserlist, review of code compiled by Babel in its online REPL, manual test with tail end of browser support)
• through some automation (diff display on pull requests, linting of output code with ESLint's es-x plugin, test with tail end of browser support, having Babel log the polyfills it'd be keen to inject...)

Why

This would give us more confidence that our configuration of , to limit the language features we use, and Babel, to transpile new features to older syntax, doesn't have gaps.

Who needs to work on this

Developers

Who needs to review this

Developers

Done when

• Plan investigation (some options in our dev catch-up metting notes (internal))