Thread class is split into a version(CoreDdoc) and other

[denizzzka]

• Thread class is split into a version(CoreDdoc) and other
• Removed outdated comments like:

///////////////////////////
/// Like this
//////////////////////////

(It was previously discussed that this is obsolete and can be removed.)

The point of this PR is to be able to further separate Thread class implementations for Posix, Windows and maybe other OSes

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[thewilsonator]

(It was previously discussed that this is obsolete and can be removed.)

Where?

[denizzzka]

@thewilsonator I think it was before 2022 and I can't find it now.
I scrupulously dragged-and-dropped these comments when I was doing some manipulations with splitting large files into smaller ones (also around the threads support), and someone commented that there was no need for this; they were planning to get rid of them anyway.

Here it also looks logical, because inside of CoreDdoc version of Thread isn't need to describe such "blocks".