Brings out the implementation of Thread class into OS-specific modules

[denizzzka]

Brings out the implementation of Thread class into OS-specific modules

(continuation of the work begun in #22805 )

This patch only separates class code. Functions that also have OS-dependend version branching are not affected for now to keep PR simple.

Also, I didn't put all mentioned here modules into a separate package osthread so as not to complicate the PR - this could be done in a separate patch.

I left the indentation unchanged at some dificult places to make it easier to compare code with its previous state - version (all) have been added wherever necessary.

[dlang-bot]

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