Add IS NOT TRUE, IS NOT FALSE, and IS [NOT] UNKNOWN

[sarahyurick]

Closes #582

Previously, sqlparser did not support "is not true," "is not false," "is unknown," and "is not unknown," so I have added them here.

[andygrove]

Thanks @sarahyurick. This seems to follow the pattern we use for other IS expressions such as IsNull / IsNotNull, so LGTM.

[andygrove]

Link to Postgres documentation on these expressions for anyone interested:

https://www.postgresql.org/docs/current/functions-comparison.html

[coveralls]

Pull Request Test Coverage Report for Build 2878598801

• 4 of 12 (33.33%) changed or added relevant lines in 2 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage decreased (-0.06%) to 85.377%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/ast/mod.rs	0	4	0.0%
src/parser.rs	4	8	50.0%

Files with Coverage Reduction	New Missed Lines	%
src/ast/mod.rs	1	77.65%

Totals
Change from base Build 2877505870:	-0.06%
Covered Lines:	9511
Relevant Lines:	11140

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💛 - Coveralls