Allow to pin modules with .jsm extension about importmap-rails

[dhh]

This explores similar territory to #57. I've been hesitant on #57, though. It feels like a lot to solve a little. Maybe we could simply allow .js(m|x)? as the extension instead. I'd find that much easier to merge.

[guybedford]

Note that .mjs is the preferred alternative extension to .js for modules and has a correctly registered MIME type unlike .jsm.

[mtgrosser]

If other filename extensions than .js are to be allowed, the list of accepted extensions needs to be taken into account in four different places:

• Directory expansion (pin_all_from), when scanning the filesystem
• Module name derivation
• Module path derivation
• Cache sweeping, during watcher initialization

#57 has been updated to provide this, while keeping the accepted extensions in one place.

From there, making the extensions configurable instead of hardcoding them adds four more lines, ensuring compatibility of import maps with the (still) existing asset pipeline.

[mildred]

The fact is that some packages out there are not using the plain .js extension, and to be able to use them we need to be able to pin other extensions.

For example chart.js main module name is chart.mjs. Once downloaded in the vendor directory, in the default configuration the pinning does not work.

[tgaff]

The preference for .mjs for modules comes from V8: https://v8.dev/features/modules#mjs It's likely widely followed.