sizebot: Combine stable and experimental results

Because we have access to the artifacts in CI, we can read bundle sizes
directly from the filesystem, instead of the JSON files emitted by our
custom Rollup plugin.

This gives us some flexibility if we ever have artifacts that aren't
generated by Rollup, or if we rewrite our build script.

Personally, I also prefer to see the whole file path, instead of just
the name, because some of our names are repeated.

My immediate motivation, though, is because it gives us a way to merge
the separate "experimental" and "stable" size results. Instead
everything is reported in a single table and disambiguated by path.

I also added a section at the top that always displays the size impact
to certain critical bundles — right now, that's the React DOM production
bundles for each release channel. This section will also include any
size changes larger than 2%.

Below that is a section that is collapsed by default and includes all
size changes larger than 0.2%.