Add placeholder types for ES6 collections

[cannorin]

Related: #5282

This PR adds placeholder types for ES6 collection types: Set, Map, WeakSet, and WeakMap.

[cannorin]

I'm not very sure why CI is failing as opam exec -- npm ci does not fail on my PC. @cristianoc could you help?

[cristianoc]

Touching Js often creates more trouble than it is legitimate to expect, for no apparent reason.
Probably something set up wrongly in the build or race condition.
I vaguely remember @cknitt sorting out a similar issue?

[cknitt]

I vaguely remember something about a duplicate module (file) that was kept in sync manually + non-deterministic build order.

[cannorin]

thanks, it looks like I had to edit jscomp/runtime/js.ml too 😄

[cristianoc]

Couple of typos in comments.

[cristianoc]

Would you update the changelog too?

[cannorin]

I'm not sure which tag to use for this. Is this a "New Feature" or "Internal"?

[cristianoc]

I'm not sure which tag to use for this. Is this a "New Feature" or "Internal"?

It think new feature as it's user visible.