feat: convert to ESM/CJS hybrid package

[yinzara]

As outlined in #1263, the NodeJS ecosystem is not yet prepared for a package as major as node-fetch being ESM only.

This PR adds cross transpilation of the 3.0.0 ESM version of node-fetch using esbuild as a prepack step before publishing and configuration in the package.json to allow for both styles of imports.

For ESM only packages that use node-fetch, they should be able to 'tree shake' the CJS files so they don't impact application size.

For CJS packages, they can continue to use node-fetch as they originally did with the understanding that in the future this support could be removed.

I have updated documentation to reflect the change.

This should allow abandonment of the 2.x branch.

[jimmywarting]

Locking this as a duplicate of #1227

There is no point to submit more variant of this solution unless you first can convince some of the maintainer that this is something we want to support.

It's not enough to argument why it should be supported and what all the benefits are. You also have to understand what the drawbacks of what this means to bundle all the things to cjs... Something which is not mentioned at all in this PR.