File name extensions for es6 modules: .js or .mjs

[jonathanolson]

From the planning doc:

( ) [DEV MEETING post-publication, Someone look into it, then decide as dev team] Should we use *.js or *.mjs in node? In the browser?
SR: See https://nodejs.org/api/esm.html for documentation about using import/export in Node (This may be experimental)
JO: Any objections to having all files end in .mjs?
CM: Would WebStorm know how to create .mjs files with .js templates. I don’t want to lose the ability to quickly create new files via templates.
JO: Let’s have someone look into WebStorm support
MK: All of my live templates are working in .mjs files without additional work
MK/JO: Let’s stay with .js for now and then batch convert to .mjs if there are benefits that are worth it
JO will create an issue and remove these notes.

I think this partially depends on Node.js support (#861). We can work around things with type:"module" top-level in package.json, so I'm not yet convinced that we need a change.

I'm going to mark as deferred for now.

[jbphet]

We will revisit this in October when Node 14 comes out, since that will have support for ES6 style modules.

[ariel-phet]

Calendar reminder added

[chrisklus]

From 11/12/20 dev meeting:

We are cool with keeping .js as our file extension. Marking this as on-hold until #861 is complete.