sv: improve support for wire and var with user-defined types

sv: improve support for wire and var with user-defined types

- User-defined types must be data types. Using a net type (e.g. wire) is
  a syntax error.
- User-defined types without a net type are always variables (i.e.
  logic).
- Nets and variables can now be explicitly declared using user-defined
  types:

    typedef logic [1:0] W;
    wire W w;

    typedef logic [1:0] V;
    var V v;

Fixes #2846