build: mjs files import other files using full paths with extensions

[IvanGoncharov]

Motivation #2277

[IvanGoncharov]

@lukejacksonn Big difference for this one and #2277 is that it adds extensions to all imports inside *.mjs files.
I don't really want to add separate *.es.js files since we already generating 4 files for every source file:

Do you see any potential problems with publishing both MJS and ESM builds as *.mjs files?

[lukejacksonn]

Well, that sure simplifies things! I like the idea.. wasn't too sure that importing .mjs still worked in the browser but I just checked and it seems like it does so I can't see there being a problem there.

[IvanGoncharov]

@lukejacksonn Thanks for trying it out 👍
There is still a chance that extensions will break some setups so treat it as an experimental feature and in the worst-case scenario, we will switch to *.es.js files.