Track build output size and fail CI if it increases too much #2936
Description
I've seen libraries do this, where they have a file saying a max allowed bundle size, and you have to increase it manually when the bundle gets bigger. This makes sure there are no accidental large bundle size increases like the one I caught in #2910 (comment). Unlike some libraries, we expect our total size to be going up regularly as we add more pages and features. That could just mean we increment the max in bigger chunks rather than incrementing it the minimum possible to let the test pass. We could also treat the root chunk separately since it's the thing that users load immediately.