Add CI to block the commit of large files

## What problem does this solve or what need does it fill?

Large files permanently bloat repo size. This sucks for contributors, and is extremely painful to fix (see #24130).

@mockersf has been a strong advocate of this, and @andriyDev and @atlv24 have both suggested blocking this in CI.

## What solution would you like?

Add a CI check that scans for large new files (over 1 MB or something?) and rejects the PR if any are detected.

The error should include clear advice on what should be done instead. Currently we're using https://github.com/bevyengine/bevy_asset_files, which is moderately cursed but does work.

This is probably fine as an on-merge check only.

## What alternative(s) have you considered?

Force push to main 😎