feat: add initial support for using mkdist

[danielroe]

use by adding the following kind of export to package.json:

  "exports": {
    "./test/*": "./dist/test/*"
  },

NOTES:

• The /* at the end guarantees we're talking about a folder of files via subpath patterns, rather than a JS entrypoint. (We aren't using subpath folder mappings as they are now deprecated.)
• Currently glob patterns are not supported (e.g. /dist/test/*.js).

TODO:

• implementation
• update readme
• add tests (pending new test fixture setup) - see test: add test fixture rather than just testing siroc package #209