new ESM implementation

This PR updates the current --experimental-modules implementation based on the work of the modules team and reflects Phase 2 of our new modules plan.

A longer form description of these changes can be found in our draft blog post.

The largest differences from the current implementation include

• packge.type which can be either module or commonjs
  • type: "commonjs":
    • .js is parsed as commonjs
    • default for entry point without an extension is commonjs
  • type: "module":
    • .js is parsed as esm
    • does not support loading JSON or Native Module by default
    • default for entry point without an extension is esm
• --type=[mode] to let you set the type on entry point. Will override package.type for entry point.
• A new file extension .cjs.
  • this is specifically to support importing commonjs in the module mode.
  • this is only in the esm loader, the commonjs loader remains untouched, but the extension will work in the old loader if you use the full file path.
• --es-module-specifier-resolution=[type]
  • options are explicit (default) and node
  • by default our loader will not allow for optional extensions in the import, the path for a module must include the extension if there is one
  • by default our loader will not allow for importing directories that have an index file
  • developers can use --es-module-specifier-resolution=node to enable the commonjs specifier resolution algorithm
  • This is not a “feature” but rather an implementation for experimentation. It is expected to change before the flag is removed
• --experimental-json-loader
  • the only way to import json when "type": "module"
  • when enable all import 'thing.json' will go through the experimental loader independent of mode
  • based on https://github.com/whatwg/html/issues/4315
• You can use package.main to set an entry point for a module
  • the file extensions used in main will be resolved based on the type of the module