C#: Replace most conversions between Variant and System.Object

This commit replaces most usages of `ConvertManagedObjectToVariant` and
`ConvertVariantToManagedObjectOfType`, by using the `Godot.Variant`
struct instead of `System.Object`.

The most notable change is to the `GetGodotPropertyDefaultValues` method
that's generated for scripts. The dictionary it returns now stores
`Godot.Variant` values.

Remaining usages are:

- The `DelegateUtils` class, for the serialization of closure display
  classes during assembly reloading by the editor. These display classes
  are compiler generated classes to store values captured by a closure.
  Since it's generated by the compiler, the only way we have to access
  the fields is through reflection. This leads to using `System.Object`.
- Converting parameters when invoking constructors from the engine.
  This will be replaced with source generators in the future.
- Legacy support for old `GetGodotPropertyDefaultValues` return values.
  We need to keep supporting the old version of this generated method
  for some time. Otherwise, if loading a project built with the previous
  version, it could lead to the loss of exported property values.
  Ideally, we should remove this legacy support before a stable release.

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