Support expression in AT TIME ZONE and fix precedence

[jmhain]

Postgres supports an expression for the timezone input to the AT TIME ZONE operator. Through some experimentation, I discovered that the precedence is actually not that of the "(any other operator)" entry in the documentation, so I corrected this as well. The latter makes the workaround from https://github.com/sqlparser-rs/sqlparser-rs/pull/1267/files no longer necessary - the test added in that PR passes without it.

Resolves #1270.

[coveralls]

Pull Request Test Coverage Report for Build 9106544276

Details

• 20 of 23 (86.96%) changed or added relevant lines in 4 files are covered.
• 3 unchanged lines in 1 file lost coverage.
• Overall coverage increased (+0.01%) to 89.508%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
tests/sqlparser_postgres.rs	6	7	85.71%
src/parser/mod.rs	7	9	77.78%

Files with Coverage Reduction	New Missed Lines	%
tests/sqlparser_common.rs	3	89.45%

Totals
Change from base Build 9060810029:	0.01%
Covered Lines:	24869
Relevant Lines:	27784

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💛 - Coveralls

[iffyio]

LGTM! cc @alamb

[alamb]

Thank you @jmhain and @iffyio