feat: Start loading contexts earlier, buffer contexts for better concurrency

[effigies]

This PR has two pieces, neither of which produce any noticeable speedup on their own. The first creates a context.loaded Promise that indicates whether the context is fully loaded. The second creates a queue that can hold onto initializing contexts until we're ready for them.

For ds000001, I was getting about 9s per run. With a queue of 5, down to 4s, and a queue of 20 got us down to 2s.

[codecov]

Codecov Report

❌ Patch coverage is 94.00000% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 87.35%. Comparing base (e208822) to head (ac133ef).
⚠️ Report is 6 commits behind head on main.

Files with missing lines	Patch %	Lines
src/schema/walk.ts	81.25%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #282      +/-   ##
==========================================
+ Coverage   87.32%   87.35%   +0.03%     
==========================================
  Files          49       50       +1     
  Lines        3620     3662      +42     
  Branches      593      600       +7     
==========================================
+ Hits         3161     3199      +38     
- Misses        449      453       +4     
  Partials       10       10              

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[effigies]

Forgot to run tests... Just validated.

[effigies]

Okay, reworked this to perform new BIDSContext() and await context.loaded in walkFileTree or in a helper async function makeBIDSContext(). As long as nobody calls new BIDSContext() directly instead of through these, the resources should get released.