[TCLCORE] Verge of Merge Warning

[Marc C. <cul...@gm...>]

This is a warning that I am on the verge of merging the mac_send branch
into main.

The branch re-implements the *send* command on macOS to allow sending to an
interpreter running in a different process from the sender.  The receiving
process is required to be owned by the same user as the sending process and
to be running on the same system.

Some key points:
* The code has been reviewed by Kevin Walzer and Steve Landers.
* I am not writing a TIP on the grounds that this is not adding any new
features; it is only implementing documented features which never got
written in the macOS port.  (I more or less followed the plan outlined in
the comments of the old tkMacOSXSend.c file.)
* The implementation uses Tk's existing mechanism for receiving DoScript
AppleEvents to do  the IPC.
* The CI tests pass.  Unfortunately I had to leave the nonPortable
constraint in place for three tests which pass reliably on my computers,
because sending the DoScript AppleEvent times out on the CI runner.

- Marc