[wasm] Enable MT in S.Linq.Parallel

[radekdoulik]

Add System.Threading.WebAssembly.Threading ref assembly to enable more parts of threading API on wasm
Fix System.Linq.Parallel assembly to work as MT on wasm
Increase the initial thread pool size to 7 to let the System.Linq.Parallel tests pass

Fixes #91661

[ghost]

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Issue Details

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Also fix the csproj condition to set the FeatureWasmThreads correctly

Author:	radekdoulik
Assignees:	-
Labels:	area-System.Linq.Parallel
Milestone:	-

[radekdoulik]

This depends on another MT areas to complete, like S.T.Monitor

[radekdoulik]

With the missing pieces in place, we pass the build.

During the test run the previously failing test passes, the next one dead locks. More to fix.

  info: [2023-12-21T16:14:44.974Z] [STRT] System.Linq.Parallel.Tests.PlinqModesTests.WithExecutionMode(labeled: ParallelEnumerable.Range, requestedDop: 4, expectedDop: 4, operation: TakeWhile+Select+ToArray, mode: Default)
  warn: Blocking on the main thread is very dangerous, see https://emscripten.org/docs/porting/pthreads.html#blocking-on-the-main-browser-thread
  info: [2023-12-21T16:14:45.990Z] [PASS] System.Linq.Parallel.Tests.PlinqModesTests.WithExecutionMode(labeled: ParallelEnumerable.Range, requestedDop: 4, expectedDop: 4, operation: TakeWhile+Select+ToArray, mode: Default)
  info: [2023-12-21T16:14:45.991Z] [STRT] System.Linq.Parallel.Tests.PlinqModesTests.WithExecutionMode(labeled: ParallelEnumerable.Range, requestedDop: 4, expectedDop: 4, operation: TakeWhile+Select+foreach, mode: Default)

[ghost]

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Issue Details

---

Also fix the csproj condition to set the FeatureWasmThreads correctly

Author:	radekdoulik
Assignees:	radekdoulik
Labels:	arch-wasm, area-System.Linq.Parallel
Milestone:	-

[radekdoulik]

Remaining work in this area, to be added in following PRs

• throw PNSE in blocking calls on the main thread and on threads with JS interop
• look at WithExecutionMode, where it is in one case blocking main thread

  info: [2024-01-10T15:52:42.288Z] [PASS] System.Linq.Parallel.Tests.PlinqModesTests.WithExecutionMode(labeled: Partitioner.Create, requestedDop: 1, expectedDop: 1, operation: Where+Select+ElementAt, mode: Default)
  warn: Blocking on the main thread is very dangerous, see https://emscripten.org/docs/porting/pthreads.html#blocking-on-the-main-browser-thread
  info: [2024-01-10T15:52:43.308Z] [PASS] System.Linq.Parallel.Tests.PlinqModesTests.WithExecutionMode(labeled: ParallelEnumerable.Range, requestedDop: 4, expectedDop: 4, operation: TakeWhile+Select+ToArray, mode: Default)