Improve PowerShell plugin's profile handling functionality

[nrusch]

Improvements to the PowerShell plugin to support profile-related settings/options.

Add support for the package_commands_sourced_first config/runtime option

Prior to this change, the rez PowerShell implementation would always source the rez context script after the user/host profile scripts, so profile-level modifications to environment variables like PATH were always squashed by the unconditional overrides in the context script.

With this change, the relative source order of the shell profile vs. the context script can be properly controlled using the package_commands_sourced_first config option, matching the behavior of SH-based shells.

NOTE: Because this option defaults to True, this also implicitly changes rez's default behavior to source the shell profile after the context when using PowerShell.

Support the norc shell plugin option.

This enables the use of rez-env --norc to bypass the sourcing of any profile scripts when using PowerShell.