Overhaul [Cleared]

1. Rename the attribute to `ClearsToDefaultAttribute` and also allow setting non-reference types (see `[RequiresBehaviourState]` for the logic to set to `default`).
1. Add a new attribute `ClearsByMethodAttribute` which takes the `nameof` the method to call on the property to clear it. This needs to handle `null` property values(!), and shall ensure the method exists on the property's type(!) and the method is accessible(!) and takes no arguments(!). The method's return value shall be ignored.