Add support for config.rollup.mjs/ESM configs

[adamburgess]

Rollup supports mjs/cjs/js extensions for the config:

# if you do not pass a file name, Rollup will try to load
# configuration files in the following order:
# rollup.config.mjs -> rollup.config.cjs -> rollup.config.js

So this adds support for that.

The change in cli.js searches for mjs/cjs with a fallback to .js (code copied from rollup)
The change in impl/ConfigLoader.js uses import when a file ends with mjs, instead of compiling and requiring.
A little extra work is needed for importing the mjs file on windows due to it needing urls and not paths: nodejs/node#34765

I would add tests but... it doesn't seem like the cli is tested. I could add an example project instead.

[PepsRyuu]

Thanks! Will give it a try as soon as I can. :)

[PepsRyuu]

Released in 0.15.2