feat: Add custom operator

[max-sixty]

From #863

• It doesn't parse anything — I'm not sure how to parse SELECT 'a' REGEXP '^[a-d]'; with REGEXP as the operator... (but fine for my narrow purpose)
• If we need tests, where would I add them?

[ankrgyl]

Apologies if I'm missing some context, but what is the purpose of this new operator? Is it so you can build up a tree manually with a custom string at a certain position?

[max-sixty]

Yes, we'd like to write:

 SELECT 'a' REGEXP '^[a-d]';

...which is how MySQL & Sqllite do regex...

[ankrgyl]

I'll add some comments on the task!

[alamb]

I think this looks good to me -- thank you @max-sixty
I took the liberty of adding a docstring to try and make the use case clearer (and avoid it being deleted by accident in the future

[coveralls]

Pull Request Test Coverage Report for Build 5005987524

• 0 of 1 (0.0%) changed or added relevant line in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.005%) to 86.16%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/ast/operator.rs	0	1	0.0%

Totals
Change from base Build 5005960670:	-0.005%
Covered Lines:	14344
Relevant Lines:	16648

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💛 - Coveralls

[max-sixty]

Thanks @alamb !

[alamb]

BTW I expect to release the next version of sqlparser to crates.io today, which will include this fix