Allow access to enum NUM_CONSTANTS field in FPP

[celskeggs]

I'm seeing engineers define enums like the following:

        enum MyEnum : U8 {
            OPTION_A
            OPTION_B
            OPTION_C
            NUM_OPTIONS
        }

Of course, NUM_OPTIONS isn't supposed to be a valid choice for this enum... but the engineer wants the ability to use the number of values in the enum in a port array definition:

        output port myPort: [MyEnum.NUM_OPTIONS] MyPort

If we were writing C++, we could use MyEnum::NUM_CONSTANTS to find the number of options, and we wouldn't need to define NUM_OPTIONS. But we can't write MyEnum.NUM_CONSTANTS in fpp; the symbol is undefined.

I'd like for us to consider allowing FPP code to access the same NUM_CONSTANTS field that is available in the generated HPP.