Add fuzzing tests to check for unhandled exceptions

For each baseline file, truncate it in various places at the start / end (to simulate a partially-typed document), run it through the e2e compiler, plus the common LSP entry points, and verify that we don't hit any unhandled exceptions.

This would probably take a **very long time**, so not sure if we'd want to do this as part of regular CI, but it would be a useful tool to catch any potential issues with handling a partially-typed document.