chore: workspace cleanups

[arendjr]

Summary

As I'm preparing to implement a Workspace watcher, I noticed a few minor code cleanups I wanted to do. I initially hit a bit of a dead end with my watcher approach, but I think the cleanups still make sense to merge to prevent a large PR down the line. Specifically:

• Moved the init_thread_pool() function, since it doesn't really seem to belong inside the scanner.
• Changed scan() to be a method on WorkspaceServer instead of a standalone function. Not really important, but seeing how the first method was a reference to the WorkspaceServer anyway, this seems cleaner. (I was going to do the same for the watcher, but that didn't work out.)
• Move the workspace tests from tests/workspace.rs to src/workspace.tests.rs. This has the advantage that we can move an inline test that is in src/workspace.rs there as well, and we only need to keep one snapshots folder instead of two. It also saves a lot of indentation in the file.
• Move the version number for static_assertions to the top-level Cargo.toml.

Test Plan

CI should remain green.

[ematipico]

Looks good!

[codspeed-hq]

CodSpeed Performance Report

Merging #5191 will not alter performance

_{Comparing arendjr:workspace-cleanups (348afda) with main (e8b32d5)}

Summary

✅ 97 untouched benchmarks