lib: expose setupInstance method on WASI class

[toyobayashi]

Background: nodejs/help#4102

wasip1-threads requires shared memory imported from host and reuse in child threads, and neither initialize nor start should be called in child threads, but WASI object also need to bind wasm instance and memory in child threads.

My current solution is so tricky without accessing setupInstance:

• https://github.com/toyobayashi/emnapi/blob/0e9d90964c9355c57a808bcd9ca88b56617fa702/packages/wasi-threads/src/proxy.ts
• https://github.com/toyobayashi/emnapi/blob/0e9d90964c9355c57a808bcd9ca88b56617fa702/packages/wasi-threads/src/wasi-threads.ts#L377-L398

[nodejs-github-bot]

Review requested:

• @nodejs/wasi

[codecov]

Codecov Report

Attention: Patch coverage is 79.16667% with 5 lines in your changes missing coverage. Please review.

Project coverage is 90.25%. Comparing base (e03af77) to head (c88140f).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
lib/wasi.js	79.16%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #57214      +/-   ##
==========================================
- Coverage   90.27%   90.25%   -0.02%     
==========================================
  Files         630      630              
  Lines      184634   184645      +11     
  Branches    36129    36129              
==========================================
- Hits       166674   166649      -25     
- Misses      11009    11023      +14     
- Partials     6951     6973      +22     

Files with missing lines	Coverage Δ
lib/wasi.js	95.26% <79.16%> (-2.51%)	⬇️

... and 30 files with indirect coverage changes

[ospencer]

From a WASI perspective, this approach seems reasonable to me. This has also come up before when comparing Node to other WASI runtimes.