feat: Convert IS TRUE|FALSE to expression

[ovr]

Hello!

Support for IS TRUE|FALSE operator was introduced in #474 by yuval-illumex, but it has a limitation.

There is a difference between IS TRUE|false and = true|false, it how it handles nulls:

CREATE TABLE test_boolean (
    col boolean
);
INSERT INTO test_boolean VALUES (true), (false), (null);
SELECT col, col IS FALSE, col = false, col is true, col = true from test_boolean;

As you can see from the output, it handles null values differently.

Thanks

[coveralls]

Pull Request Test Coverage Report for Build 2370698249

• 14 of 15 (93.33%) changed or added relevant lines in 3 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage increased (+0.005%) to 89.822%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
tests/sqlparser_common.rs	8	9	88.89%

Files with Coverage Reduction	New Missed Lines	%
tests/sqlparser_common.rs	1	97.01%

Totals
Change from base Build 2367540025:	0.005%
Covered Lines:	8322
Relevant Lines:	9265

---

💛 - Coveralls

[alamb]

I am not a huge fan of the growing number of Expr::Is* variants, but that is not something introduced in this PR.

[alamb]

It is somewhat unfortunate we don't have Expr::Is(Box<Expr>, Box<Expr>) which would handle all these cases.

What do you think @ovr ? Should I file a ticket to try and reduce the duplication in Expr?

[ovr]

IMHO: It's a breaking change, which doesn't give any advantages over this. Right now, it's really easy to implement a match pattern over this because names are really helpful.

[alamb]

@ovr github tells me there is a conflict to resolve in this PR -- I haven't been able to fix such issues in the past as I don't have the ability to push commits to your PR

Can you please resolve the conflicts, and then I will merge this PR?

Thank you!

[ovr]

Rebased ✅

[alamb]

Thanks @ovr