feat: parse postgres trigram operators

[lu-zero]

See https://www.postgresql.org/docs/current/pgtrgm.html

Fixes #1180

[lu-zero]

It is missing tests for now.

[gvozdvmozgu]

Let me know if you need any help with it!

[lu-zero]

It seems to not be enough, so if you can spot what I'm missing would be great :)

[lu-zero]

Thank you :)

[benfdking]

Sorry I've been a little slow to get to this change.

I think it's great and thanks for the contribution. The thing I am slightly worried about is that unlike sqlfluff we shouldn't make extensions part of the default dialect.

I think it should add the concept of modifiers, I'll have a look at doing this next.