[Feature Request] Regex-based mappings

[qwtel]

It would be very nice if something like this was supported:

mappings: {
  "https://esm.sh/(@?[^@]+)@([^/]+)/(.*).js": {
    name: "$1",
    version: "^$2",
    subPath: "$3.js",
  },
},

[dsherret]

More discussion in https://github.com/denoland/dnt/issues/308