Rename ESM output to .mjs

[geelen]

I've just been playing around with the @std/esm loader, which makes reference to the fact that Node's ES6 module support is going to be tied to the file extension *.mjs. I admit I don't know much context around this issue but I saw this and thought it might make sense to follow the new convention.

[coveralls]

Coverage remained the same at 94.682% when pulling 4b6fb9b on geelen:patch-1 into 3d47b59 on mobxjs:master.

[mweststrate]

@rossipedia could you review this one?

[rossipedia]

@geelen there's another minor change you'll need to make: pkg.module would also need to change to mobx.mjs.

I'm not 100% on the ramifications of this change at the moment, either. While ideally moving to .mjs is the Right Thing™ to do, and at least Webpack handles that extension currently, I'm not sure how many other bundlers / ecosystems would get tripped up by this. Although, to be fair, we gotta start somewhere. As far as my personal opinion goes, though, I'm all for it 👍

[capaj]

I personally dislike mjs, but it is the standard for node.js and browsers don't really care for it. So makes sense to have mjs instead of js.
IMHO all other bundlers should be allright, but people could run into issues if they have the file whitelisted/blacklisted somewhere. Would be best to release in 3.3.0 instead of in 3.2.3

[coveralls]

Coverage increased (+0.004%) to 94.686% when pulling 3da0318 on geelen:patch-1 into 3d47b59 on mobxjs:master.

[mweststrate]

Thanks & merged. Will be part of next release. Probably blowing up in somebodies face like any module / bundling related change ever did in the past 😢

[mweststrate]

Can somebody confirm this will work on React Native as expected as well?

[mweststrate]

React native does not support .mjs extension yet. Also, webpack doesn't support it yet (will be added in 4). See: webpack/webpack#5686

Rolled back this PR and created a fresh follow up issue: #1172

[geelen]

Also, since I posted this PR, Node has gotten experimental support for modules which... I don't think should change things but who knows!

Probably blowing up in somebodies face like any module / bundling related change ever did in the past

That's my experience too. Good luck!