Specify resolving sources

[jridgewell]

I had a bug in my source URL resolution where I resolved sourceMappingUrl and sourceRoot, then used that result to resolve source. The under-specified behavior is that sourceRoot is prepended (treating it as a directory) to source, then resolved using sourceMappingUrl as the base.

Implementations:

• source-map: computeSourceURL and join
• Chrome: parseMap and completeURL

Open questions:

• What should the algorithm return when source = null?
  • Chrome treats it as "null"
  • source-map treats it as ""
• Handling of data: URIs (the basic URL parser will throw an error).
  • Chrome returns the prefixed-source href
  • source-map throws
• Other special schemes like blob
  • https://source.chromium.org/chromium/chromium/src/+/main:third_party/devtools-frontend/src/front_end/core/common/ParsedURL.ts;l=368-371;drc=081ba89b58575aae4bdb8fb84f71375c53bbb82e

[hybrist]

Should this PR be ported to https://github.com/tc39/source-map instead, now that the spec has moved?