Add flag to stop printing to stdout/stderr

[vnen]

This allows the terminal output to be suppressed but still be captured by print/error handlers. Which is useful for a GDScript test suite (cf. #41074).

[Calinou]

Isn't there a disable_stdout project setting for that already (and the --quiet command line argument)? We should harmonize this eventually.

[vnen]

Isn't there a disable_stdout project setting for that already (and the --quiet command line argument)? We should harmonize this eventually.

@Calinou this disables printing completely which means the print handlers never get called. What I want here is to still have print handlers but don't output to actual stdout/stderr.

[vnen]

Changed to add the flag to OS instead of a global one. This only concerns OS in any case, since it's the one actually doing the printing.

[akien-mga]

Thanks!