[Feature] Add env variable to enable rich output when it is off by default

[corneliusroemer]

Description

Using app = typer.Typer(pretty_exceptions_enable=False) I can disable rich exceptions.

However, sometimes it would be nice to have them switched on. I like that you can switch off exceptions using the env variable _TYPER_STANDARD_TRACEBACK=1. It would be great if there was an equivalent _TPER_RICH_TRACEBACK=1 that would turn rich tracebacks on even if the script is configured with pretty_exceptions_enable=False