Support setupProxy.mjs

Created by: ulrichb

Is your proposal related to a problem?

Now as Node.JS 13.2 un-prefixed ES 6 Modules, I'd like to use ES 6 imports in my setupProxy.mjs, but CRA forces me to use setupProxy.js.

Describe the solution you'd like

Next to setupProxy.js, allow to use setupProxy.mjs (or setupProxy.cjs).

See also https://nodejs.org/api/esm.html#esm_enabling.