1279 add support for 3rd party prompt implementations in pwsh powershell

[instinct-vfx]

This PR adds support for setting of nested prompt decorators and for 3rd party prompts by adding an option to only set the prompt as an environment variable so it can be used in 3rd party prompts.

To support unicode setups (as in unicode prompt decoration) i also had to add unicode support to the powershell_base write operation in spawn_shell.

I also changed the default for the prompt in Powershell as the old default is really weird when using it repeated for nested shells.

Behavior for nested shells with new defaults:

Example with custom unicode setup in oh-my-posh:

It can even do Beer emojis ❤️

Fixes #1279

[instinct-vfx]

Consider this WIP until i get to fix the 2.7 compatibility issues.

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[instinct-vfx]

Updated powershell base to support unicode output in both py2 and py3. It's likely not the most elegant way, but the least invasive one that i found.