implement a shutdown protocol

[mmomtchev]

Avoid using Worker.terminate() since since there seems to be an issue in recent Node.js versions when using WASM in a worker_thread - it can cause an infinite loop in node::FreeEnvironment.

The problem is much more frequent since Node.js 18.18 on macOS and Windows but it seems present - albeit very rare - even on earlier versions and on Linux.

Fixes #65

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 89.13043% with 5 lines in your changes missing coverage. Please review.

Project coverage is 75.00%. Comparing base (c97a34b) to head (4cd1ee1).
Report is 180 commits behind head on main.

Files with missing lines	Patch %	Lines
src/index.ts	78.94%	4 Missing ⚠️
src/vfs-http.ts	75.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #67      +/-   ##
==========================================
+ Coverage   74.74%   75.00%   +0.25%     
==========================================
  Files           8        8              
  Lines        1964     2000      +36     
  Branches      204      210       +6     
==========================================
+ Hits         1468     1500      +32     
- Misses        489      493       +4     
  Partials        7        7              

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