WeakPool: GC pool based on weak.Pool

[arnodel]

Since go 1.24 weak pointers are available via the weak stdlib package. This means we can now have a GC pool with similar characteristics as luagc.UnsafePool but implemented safely in Go. We get further simplifications by using runtime.AddCleanup instead of runtime.SetFinalizer.

We could remove all other luagc.Pool implementations in the future I think but for now that would mean we require go1.24 (which is a big just from go1.18 currently)

[codecov]

Codecov Report

❌ Patch coverage is 81.55340% with 38 lines in your changes missing coverage. Please review.
✅ Project coverage is 67.36%. Comparing base (63a9cb6) to head (4470ace).
⚠️ Report is 25 commits behind head on lua5.5.

Files with missing lines	Patch %	Lines
runtime/internal/luagc/registry.go	16.66%	20 Missing ⚠️
runtime/weakpool.go	95.12%	7 Missing and 1 partial ⚠️
runtime/runtime.go	22.22%	5 Missing and 2 partials ⚠️
runtime/internal/luagc/clonepool.go	50.00%	0 Missing and 1 partial ⚠️
runtime/internal/luagc/safepool.go	50.00%	0 Missing and 1 partial ⚠️
runtime/internal/luagc/unsafepool.go	50.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           lua5.5     #123       +/-   ##
===========================================
- Coverage   80.79%   67.36%   -13.43%     
===========================================
  Files         168      170        +2     
  Lines       15144    15602      +458     
===========================================
- Hits        12235    10511     -1724     
- Misses       2549     4756     +2207     
+ Partials      360      335       -25     

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