extension! esm setting should support defining a specifier

[bartlomieju]

Currently esm property in extension! macro support syntax like:

  esm = [
    dir "polyfills",
    "00_globals.js",
    "02_init.js",
    "_brotli.js",
  ]

This creates a specifier like ext:<extension_name>/<dir>/<module_name>. In some cases we want to be able to explicitly set the extension for a module, @mmastrac suggests syntax like:

  esm = [
    dir "polyfills",
    "00_globals.js",
    "02_init.js",
    "_brotli.js",
    "foo.js" as "node:foo" 
  ]

[bartlomieju]

Actually I'm not sure this is worth it, the current solution with two separate fields (esm and esm_with_specifiers) works fine and seems less confusing. Closing for now.