Add getCursorPos() to std/terminal

[MatrixManAtYrService]

This would be handy for making terminal apps which display content below the prompt (e.g. fzf does this).

Need to test it on windows before I remove "draft" status.

[juancarlospaco]

Documentation comment should mention that internally it writes to stdout IMHO.

[MatrixManAtYrService]

Oof, I just realized that it doesn't behave very well if there's not a terminal in the loop (writes the request to stdout and blocks while waiting for something from stdin). It might do to warn users about that. Added a comment.

[MatrixManAtYrService]

From the failing CI:

D:\a\Nim\Nim\lib\pure\terminal.nim(226, 24) Error: getCursorPos(getCurrentProcess()) can raise an unlisted exception: ref OSError
FAILURE
Error: Process completed with exit code 1.

Super sophisticated test program:

# test.nim
import terminal
echo getCursorPos()

Placing it in the same dir as terminal.nim and running nim c test.nim replicates the error, and it goes away after adding OSError to the list. The test program fails, however:

PS C:\Users\matt\src\Nim\lib\pure> .\test.exe
C:\Users\matt\src\Nim\lib\pure\test.nim(2) test
C:\Users\matt\src\Nim\lib\pure\terminal.nim(225) getCursorPos
C:\Users\matt\src\Nim\lib\pure\terminal.nim(221) getCursorPos
C:\Users\matt\.choosenim\toolchains\nim-2.0.0\lib\std\oserrors.nim(92) raiseOSError
Error: unhandled exception: The handle is invalid.
 [OSError]

Fixed it like so:

  proc getCursorPos*(): tuple [x, y: int] {.raises: [ValueError, IOError, OSError].} =
    return getCursorPos(getStdHandle(STD_OUTPUT_HANDLE))

Tested on windows, works ok.

[github-actions]

Thanks for your hard work on this PR!
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