Allow more locations when installing custom PSES

[ForNeVeR]

Currently, when choosing a custom PSES, we expect the user to select the root folder that contains PowerShellEditorServices/PowerShellEditorServices.psd1

This is rather confusing in practice.

Let's allow the users to choose any of:

• the directory containing PowerShellEditorServices/PowerShellEditorServices.psd1
• the directory containing PowerShellEditorServices.psd1
• any file in the directory containing PowerShellEditorServices.psd1

We should auto-discover the PSES instance from any of these locations.