Add support in IS boolean filter

[yuval-illumex]

The following query is valid and should be supported:
SELECT f FROM foo WHERE field = true

Verified on latest postgres version

[coveralls]

Pull Request Test Coverage Report for Build 2232785342

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

• For more information on this, see Tracking coverage changes with pull request builds.
• To avoid this issue with future PRs, see these Recommended CI Configurations.
• For a quick fix, rebase this PR at GitHub. Your next report should be accurate.

Details

• 12 of 15 (80.0%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.005%) to 90.408%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/parser.rs	6	9	66.67%

Totals
Change from base Build 2219792542:	0.005%
Covered Lines:	8049
Relevant Lines:	8903

---

💛 - Coveralls

[alamb]

👍 looking good

[alamb]

Looks good -- thank you @yuval-illumex