Convert package to be pure ESM

[fregante]

The minimum version can be increased to Node 12 and the CJS build can be dropped. The ESM build is still necessary to tree shake the tests however.

AVA configuration should be copied from https://github.com/sindresorhus/refined-github/blob/main/package.json

It’s probably best not to update dependencies in the same PR unless necessary because they’re likely to compound issues.