TODO: mjs and cjs file extension support

[3cp]

Nodejs v12 has brought support of esm format.

There are two added file extensions: mjs and cjs.

1. mjs is treated as esm module.
2. cjs is treated as commonjs module.
3. js is treated as either esm or commonjs module based on nearest package.json "type" field.

While dumber doesn't care which module format the file is (dumber uses AST to find out), we still need to add the file extensions.

• add file extensions to dumber-module-loader's known extensions, add extensions to nodejsIds helper.
• cannot assume nodejs file always ends in .js, also need to update nodejs module resolution algorithm to cover new file extensions.

https://nodejs.org/dist/latest-v12.x/docs/api/esm.html

Need to support the new "exports" field in package.json defined in the above doc.

[3cp]

This is very odd...

Mandatory file extensions
A file extension must be provided when using the import keyword. Directory indexes (e.g. './startup/index.js') must also be fully specified.

[3cp]

Basic support is done.
But nodejs v12 esm comes with exports #17.