Support the rarer extension equality operators

[jaybosamiya]

Warning

We are considering removing !~= and !~~= operators from Verus, thus we might not wish to merge this PR.

This PR adds support for the rarer extensional equality operators (!~= and !~~=, which are the evil negation twins of =~= and =~~= respectively; see verus#23ac0b0).

Closes #145.

^{By submitting this pull request, I confirm that my contribution is made under the terms of the MIT license.}

[parno]

This fixes the issue, but are we sure Verus should be supporting such things? I didn't realize Verus included them, and they're pretty ugly.

[jaybosamiya-ms]

I don't particularly like these operators either, tbqh (you'll notice that I call them the "evil negation twins" for a reason 🙃). I too hadn't realized that Verus had included them until I went searching for them. They've existed for ~2 years, and the fact that no one has actually hit this issue until recently for verusfmt implies that no one is actively using them. If folks want to pull this out of Verus, and we thus don't merge this into verusfmt, that would be fine by me.

[parno]

There was some support at the Verus meeting today for removing those operators, so perhaps we should hold off merging this for now.

[jaybosamiya-ms]

Marked as draft so it doesn't look like an approved PR that is just waiting to be merged. Once it these operators are ripped out of Verus, we should close this PR and close #145.