Previously, we always just used `int` when dealing with menu values on the C++
side. It's currently the only type where we have the same base type (`int`) for
two different socket types (integer and menu sockets). This has some downsides:
* It requires special cases in some places.
* There is no function from static base type to socket type (which is useful for
some utilities like `SocketValueVariant`).
* It implicitly allows operations on menu values that shouldn't be allowed such
as arithmetic operations and conversions to and from other types.
This patch adds a new `MenuValue` type that is used for menu sockets in Geometry
Nodes and the (CPU) Compositor, clarifying the distinction between integer and
menu values.
Pull Request: https://projects.blender.org/blender/blender/pulls/144476
c90a137a27