Block direct modification of Value-property in configuration options

[fflaten]

PR Summary

Makes the setter of Value-property in configuration options private. This is done because modifying Value directly won't flip the internal _isOriginalValue-flag used to detect whether a user has modified the option. This is important to know which values to override when merging configuration.

Fix #1971

Possible breaking change: Users may create their advanced configurations by modifying Value directly because it has been possible or because it can be easier in certain scenarios, ex.

$c.Debug.WriteDebugMessagesFrom.Value += 'MyForgottenCategory'
# vs after PR
$c.Debug.WriteDebugMessagesFrom = $c.Debug.WriteDebugMessagesFrom.Value + @('MyForgottenCategory')

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