who: implement for Windows

[wtcpython]

who was previously unavailable on Windows.

This adds a backend backed by WTS session information and enables it in the Windows feature set. Option parsing and output formatting remain shared with the Unix implementation.

[Copilot]

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[codspeed-hq]

Merging this PR will not alter performance

✅ 367 untouched benchmarks
⏩ 50 skipped benchmarks¹

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_{Comparing wtcpython:who-windows (786338f) with main (c9077c7)}

Footnotes

1. 50 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/date/date-locale-hour (fails in this run but passes in the 'main' branch)
Skip an intermittent issue tests/tail/pid-pipe (fails in this run but passes in the 'main' branch)
Skip an intermittent issue tests/tail/tail-n0f (fails in this run but passes in the 'main' branch)
Skipping an intermittent issue tests/cut/bounded-memory (passes in this run but fails in the 'main' branch)
Note: The gnu test tests/csplit/csplit-heap is now being skipped but was previously passing.
Note: The gnu test tests/cut/cut-huge-range is now being skipped but was previously passing.
Note: The gnu test tests/expand/bounded-memory is now being skipped but was previously passing.
Congrats! The gnu test tests/seq/seq-epipe is now passing!

[oech3]

If it is actually not supported, this code should be removed. We don't suppprt building stubs.

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