fix: windows build

[vasco-santos]

This PR fixes the ESM build in windows.

There were two issues with the specificities of paths in Windows that were not allowing windows builds:

• File URL path must not include encoded \ or / characters
• Cannot import ./indexjs from /esm/_ipjsInput.js

The first was basically because we were creating the following type of URL, which was problematic on its encodings.

URL {
  href: 'file:///C:%5CUsers%5CRUNNER~1%5CAppData%5CLocal%5CTemp%5Cf0cb966061a76cc867b77cbb3dd07e18/package.json',
  origin: 'null',
  protocol: 'file:',
  username: '',
  password: '',
  host: '',
  hostname: '',
  port: '',
  pathname: '/C:%5CUsers%5CRUNNER~1%5CAppData%5CLocal%5CTemp%5Cf0cb966061a76cc867b77cbb3dd07e18/package.json',
  search: '',
  searchParams: URLSearchParams {},
  hash: ''
}

Error:

TypeError [ERR_INVALID_FILE_URL_PATH]: File URL path must not include encoded \ or / characters

The second error was due to the windows relative path obtained in windows came as './src\\index.js', meaning the generated code created was:

import './indexjs' from `./esm/_ipjsInput.js`

Resolves #15

[vasco-santos]

An example of a build working on Linux, MacOS and Windows with this PR: ipfs/aegir#865

It would be great to extend the tests here to support Windows and avoid regressions, but https://github.com/mikeal/estest does not support windows. We should open an issue to add such tests as follow up