Add support for `undef

[gjcoram]

[root@5033e3bcb6f9 io]# target/release/openvaf r3_cmc.va
error: macro 'undef' has not been declared --> /io/r3_cmc.va:127:9 | 127 | undef VAMS_COMPACT_MODELING
| ^^^^^^ macro not found here

error: unexpected token identifier; expected 'discipline', 'nature' or 'module'
--> /io/r3_cmc.va:127:16
|
127 | `undef VAMS_COMPACT_MODELING
| ^^^^^^^^^^^^^^^^^^^^^^^^^ expected 'discipline', 'nature' or 'module'

error: could not compile r3_cmc.va due to 2 previous errors

[gjcoram]

Also `resetall is not supported.

[gjcoram]

Actually, per IEEE Std 1364:
The text macro facility is not affected by the compiler directive 'resetall.
I had been under the impression that it was supposed to undefine all macros, but that was incorrect. The things it affects are not generally relevant to compact models, so OpenVAF can probably just ignore it.

[gjcoram]

I checked in changes to support undef and ignore resetall into my fork. Let me create a pull request.