Ensure we're properly supporting ESM in Node

[benlesh]

See the write up by @Rich-Harris here:

d3/d3#3469

[benlesh]

(cries internally)

[tomastrajan]

It would be great (and compliant) if rxjs shipped

dist/esm/*.mjs

instead if

dist/esm/*.js

(ie use .mjs extension, but only in esm folders)

The reson for this is that some tooling assumes (correctly) that .js means it's .cjs (in the absence of "type": "module" in the nearest package.json) and breaks and there is no way to override this behavior?

[dynst]

Dual-packaging properly is basically impossible just using tsc as the transpiler, because it always outputs .js extensions no matter what. The new observable package uses tshy, so maybe it's worth switching to using that.

Because it looks like the problem this old PR was trying to fix still exists (all ESM files need to use the .mjs extension, but they currently don't): #7130

https://nodejs.org/api/packages.html#determining-module-system

The reason no one's noticed is the ESM files aren't actually exported in Node: #7418