Mandatory extension support for builds

[ismail-syed]

Description

This PR update our build system to support mandatory file extensions. While exploring snowpack and swc, we hit issues where those respective bundlers couldn't parts our mjs and esnext builds. This PR fixes that.

This PR fixes the following errors we get when running quilt packages in a webpack 5 react app:

Module not found: Error: Can't resolve './build/esm/index' in 'node_modules/@shopify/react-form'"

🎩

• yarn run clean && yarn run build to run a fresh build

• Peek into a esm build to verify the .mjs extensions (eg. packages/react-html/build/esm/common.mjs)
  

• Peek into an esnext build to verify the .esnext extensions (eg. packages/react-html/build/esnext/common.esnext)
  

• Peek into a cjs build to verify the relative .js extensions (eg. quilt/packages/react-html/build/cjs/common.js)
  

Checklist

• I have added a changelog entry, prefixed by the type of change noted above (Documentation fix and Test update does not need a changelog as we do not publish new version)

[BPScott]

Haven't thumbed through every package but it looks like sk is now indeed providing full paths!