Add support for 3rd party prompt implementations in pwsh / Powershell

[instinct-vfx]

The current implementation to set the prompt on windows creates a custom prompt function. This overrides existing prompt functions a user may have set including 3rd party prompts like https://ohmyposh.dev/

If a user has his prompt customized, dropping himself into a rez created shell removes all of the user's setup.

I would suggest adding a new PowershellBase setting that allows switching between actually setting a prompt function (the current behavior) to setting it as an environment variable only (so it can be used by 3rd party prompts easily).

PR incoming soon unless someone objects

[instinct-vfx]

For the record, while starting on this i realised that the current implementation does not support nesting at all. Had not occured to me before which is kind of weird. So adding support for nested prompts too.