[FIRRTL][CAPI] Add functions for property types

[SpriteOvO]

This PR adds FIRRTL C-API functions for property type, which are necessary for chipsalliance/chisel#3626.

I found that the port direction enum can be reused in the FIRRTLClassElement struct, so I renamed the enum and enumerations for that purpose. Since Chisel is probably the only user for now, I think the API breaking change might not be a big deal?

[mikeurbach]

The new APIs for property types look good to me.

I found that the port direction enum can be reused in the FIRRTLClassElement struct, so I renamed the enum and enumerations for that purpose.

This makes sense to me.

[SpriteOvO]

@mikeurbach Thanks for your review ^^. Typo has been fixed and branch has been rebased, waiting for CI done.