bpo-37354: Make Powershell Activate.ps1 script static to allow for signing (GH-14967)

- Remove use of replacement text in the script
- Make use of the pyvenv.cfg file for prompt value.
- Add parameters to allow more flexibility
- Make use of the current path, and assumptions about where env puts things, to compensate
- Make the script a bit more 'idiomatic' Powershell
- Add script documentation (Get-Help .\.venv\Scripts\Activate.ps1 shows PS help page now