Migrate custom functions available in the dynamic runspace to a startup script & add unit tests

[michaeltlombardi]

Describe the Change You Would Like

The custom functions available in the dynamic runspace used via Pwsh::Manager.execute() be moved to their own separate script file and become unit tested.

While this work is not blocked on #132, being able to specify the script as part of the InitialSessionState would further simplify the init script. In the meantime, the file could be read from disk and loaded as a string, similar to how it is currently implemented.