[work-in-progress][API] RuntimeFeature.IsMultithreadingSupported

[pavelsavara]

Fixes #77541
Continuation of #123329

Mostly renames

• internal Thread.IsSingleThreaded -> public RuntimeFeature.IsMultithreadingSupported
• internal Thread.ThrowIfSingleThreaded() -> internal RuntimeFeature.ThrowIfMultithreadingIsNotSupported()
• PlatformDetection.IsThreadingSupported -> PlatformDetection.IsMultithreadingSupported
• PlatformDetection.IsNotWasmThreadingSupported -> PlatformDetection.IsNotMultithreadingSupported

[Copilot]

Pull request overview

This pull request introduces a new public API RuntimeFeature.IsMultithreadingSupported to replace the internal Thread.IsSingleThreaded property. This is part of API proposal #77541 to provide a public feature flag for determining if multithreading is supported on the current platform (particularly relevant for single-threaded WebAssembly environments).

Changes:

• Introduces public RuntimeFeature.IsMultithreadingSupported property and internal ThrowIfMultithreadingIsNotSupported() method
• Removes internal Thread.IsSingleThreaded and Thread.ThrowIfSingleThreaded()
• Renames PlatformDetection.IsThreadingSupported → IsMultithreadingSupported and IsNotWasmThreadingSupported → IsNotMultithreadingSupported
• Updates hundreds of test files to use the new naming convention
• Configures the feature switch in MSBuild targets for WebAssembly

Reviewed changes

Copilot reviewed 272 out of 272 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
System.Runtime.cs	Adds public API surface for IsMultithreadingSupported
RuntimeFeature.cs	Implements new property and internal throw helper
Thread.cs	Removes old IsSingleThreaded API
PlatformDetection.cs (multiple)	Updates to use RuntimeFeature instead of environment variable
Environment.cs	Uses new API for ProcessorCount logic
Microsoft.NET.Sdk.WebAssembly.Browser.targets	Configures feature switch for WASM
tests.browser.targets	Removes IsBrowserThreadingSupported environment variable
200+ test files	Updates ConditionalFact/Theory attributes to use new naming

[Copilot]

Pull request overview

Copilot reviewed 273 out of 273 changed files in this pull request and generated no new comments.

[jkotas]

IsThreadingSupported -> IsMultithreadingSupported

Can you do 1 boring prep PR that has just this rename and nothing else, and do the rest in this PR? It is hard to find any interesting changes in 250+ changed files.

[teo-tsirpanis]

Suggested change

	nameof(IsDynamicCodeCompiled) => IsDynamicCodeCompiled,
	nameof(IsMultithreadingSupported) => IsMultithreadingSupported,