Enable additional plugins to be added via $additionalPlugins

[JustinGrote]

PR Summary

Adds support for a $SCRIPT:additionalPlugins to be defined to allow outside modules to contribute their own plugins via this method:

& (gmo pester) {$f = New-PluginObject etc..; $SCRIPT:additionalPlugins = @($f)

See: #2011 (reply in thread)

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• Tests are added/update (if required)
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[nohwnd]

This would not allow you to add a plugin in the middle of our built-in plugins, or remove other plugins, hope that is okay for your usecase. You could use a list of scriptblocks, that would take list of plugins and give you another list, but that gets complicated fast.

Please add a note that this is a quick hack for test adapter, and that no-one should depend on it. And if they do then they should let us know in this issue: #2021 to help drive the future plugins public api.