estrella doesn't seem to be working within ESM context

As more and more dependencies are updating to ESM, I’ve figured it’s time to bite the bullet and make my project ESM-ready as well.

But adding "type": "module" to my project’s package.json leads to these two issues when running node build.js:

1.   import {build, cliopts} from "estrella";
             ^^^^^
     SyntaxError: Named export 'build' not found. The requested module 'estrella' is a CommonJS module, which may not support all module.exports as named exports.
     CommonJS modules can always be imported via the default export, for example using:
     
     import pkg from 'estrella';
     const {build, cliopts} = pkg;
   

   the proposed workaround works though.

2. TypeError: Cannot read properties of undefined (reading 'filename') this is due to this code:

   https://github.com/rsms/estrella/blob/e5b985bd3292ddaa87022b97062be2f18b15b182/src/util.js#L27

   process.mainModule does not seem to exist in ESM.

[mhamann]

I proposed a fix via #53. it works locally for me and all tests pass.

[mhamann]

I’m experiencing the second issue (TypeError: Cannot read properties of undefined (reading 'filename')) as well on a project that worked fine with v1.3.x.

I need 1.4.x because I’m building a browser JS bundle and without the sourcesContent option, my sourcemaps are broken.

(@ptim ironically, I was using the same template for my use case)