Define decoding algorithms for source maps

This PR introduces "decoded source map" data structures, which are
internal spec representations of the information encoded in source maps.

It also defines algorithms to decode source maps from either a JSON
string or [infra](https://infra.spec.whatwg.org/) representation.

The goal is:
- use them to explicitly write down all the possible error cases
- use them as a starting point to define new data structures, for
  example for the scopes proposal
- eventually add algorithms such as "get the original location given a
  decoded source map and a generated location".

This PR also explicitly defines sources/sourceRoot resolution in terms
of the [WHATWG URL](https://url.spec.whatwg.org/) spec.