[sizebot] Add link to diff view (#24790)

Updates the sizebot output so that the file names link to a diff view of
the corresponding build artifact.

Example diff view: https://react-builds.vercel.app/commits/955cad9bcc6d755b2a672f8038fe9754e0fe5108/files/oss-stable-semver/react-dom/cjs/react-dom.production.min.js?compare=c3d7a7e3d72937443ef75b7e29335c98ad0f1424

The diff view itself is rendered by a Next.js app that I built as a side
project and is hosted at https://react-builds.vercel.app. If we find
this useful enough I could move the app to a React-owned repo but since
this isn't a critical feature it might be OK to leave it separate for
now, so we don't need to commit to supporting it indefinitely.