v0.0.0-5096-gdc81bac3c

Add support in VAST for various SystemVerilog constructs.

- Typedefs
- Enums
- Structs
- Arrays with a specified element type
- Arrays with an expression that is the max rather than the width
- Return statements
- More complete parameter and argument declarations
- Literals that explicitly declare width and signedness
- Power operator
- Remove the prohibition on DataType::Emit(), since it now needs to be emitted in contexts without an identifier.

PiperOrigin-RevId: 634178542