support mock injections usable to mock globalThis value

[iambumblehead]

closes #198

@koshic @Swivelgames and/or @tommy-mitchell please review.

Changes here allow esmock to be called this way. esmock adds an 'import' statement at the top of the file, importing the mock-versions of the definitions,

const mockedReq = await esmock('../src/req.js', {
  import: {
    fetch: () => '[{"name":"jim","id":1},{"name":"jen","id":2}]'
  }
})

await mockedReq()
// '[{"name":"jim","id":1},{"name":"jen","id":2}]'

req.js

import { fetch } from 'file:///esmockDummy.js' // this line added by esmock

export default async url => fetch(url + '?limit=10')

[iambumblehead]

very want to merge this

[koshic]

@iambumblehead look ok for first time implementation, thx!

But there are obvious pitfalls with code directly uses globalThis, because 'globalThis.fetch === fetch' returns false after injection. So we have to patch globalThis object.

[iambumblehead]

Maybe it will become necessary to mock "globalThis" specifically and maybe not. This PR allows one to write unit-tests around fetch and other globals. It also enables importing mock values in creative ways, to alter the way a script might use "globalThis".

[iambumblehead]

thanks for the approval will merge and publish sometime over the next day or so