Allow to customize output extension

[egoist]

To have both ES module and CommonJS module co-exist in a Node.js package, we need to use different extensions, https://nodejs.org/api/esm.html#esm_enabling

[evanw]

I added a new flag for this: --out-extension:.js=.mjs (as well as JavaScript and Go APIs). It's a mapping instead of a single value because I plan to have more file types in the future such as CSS and HTML, and you may want to customize those output extensions too. This was published in esbuild version 0.6.6.

[tonyxiao]

Is it possible to transform the file extension in require statement too? Otherwise when I try to transform esm into cjs the require statement still says require('*.js') which would of course fail if the out extension has been changed to *.cjs