Add a way to show where top level eval is in progress

[IanButterworth]

It would be helpful to be able to see where top level eval is at during a running script or test suite, for debugging slow execution/hangs. Especially on remote machines that are harder to send a SIGUSR1/SIGINFO to.

This adds --show-eval=[yes|no|loc]. yes is the full code with location info, loc is just the main expr location which should be helpful-enough.
For example --show-eval=loc (which also is what --show-eval with no args enables)

julia> Pkg.test("Random", julia_args=`--show-eval=loc`)

With this implementation information is limited when test code is within @testsets or scripts which are mostly a big function that is called at the end. Perhaps more can be done to give info in those cases?

Also maybe this should be --trace-eval to go alongside --trace-compile and --trace-dispatch.

[IanButterworth]

I'm thinking julia-actions/julia-runtest could be taught to enable this when github actions debug logging is enabled

[DilumAluthge]

Seems like this could also be useful in other use cases, e.g. running a long-running HPC job on a computer cluster where you don't have direct access to the process (and thus can't send SIGUSR yourself.

[Seelengrab]

Should/could this use a carriage return instead of a hard newline or something else that's a bit smarter than just printing a new line? It feels like enabling this would be very spammy for longer testsuites.

[IanButterworth]

It's a nice idea, but I'd worry about printing over other prints. Is there a way to determine whether anything has been printed to stdout since the last print here? Some sort of io position value?