Redo package size changes checks in CI

[jakebailey]

We used to get this via the LKG task.

After #52226, LKG no longer exists. To ensure that we can still get some idea of when we accidentally regress package size too much, I had to add some hacks to the smoke test in CI which builds LKG at main, saves it outside the tree, then switches back to the PR and copies lib back, as though it were there. Then our old package size checks can kick in.

This is a hack and just temporary. What we should actually do is introduce a new CI task which checks this explicitly, then eliminate size checks from our LKG task itself.

The best thing would be to create some failure threshold in CI (right now, 10%?), that fails the build. But, using GitHub's checks API, we could feasibly also output a markdown report into the UI to look at. (This is where I'd like to stick perf results and other changes in the future.)

I've looked at the existing actions in the marketplace, and none really do what we want, in that they all reply via a comment, which will be noisy. Not sure what to do about that, besides writing a whole new action from scratch. Maybe that's fine, because there are other actions I'd like to write too (e.g. the errors delta repo could be an action that runs on every PR, as could DT and perf).

See also #53123.