feat: Support PostgreSQL exponentiation.

[michael-2956]

As of current version, SELECT 3 ^ 4 in the PostgreSQL dialect reads as BinaryOperator::BitwiseXor, while it should read as the exponentiation operation. I added BinaryOperator::PGExp for that purpose and completed all the tests in this pull request.

[coveralls]

Pull Request Test Coverage Report for Build 4312856330

• 16 of 17 (94.12%) changed or added relevant lines in 4 files are covered.
• 827 unchanged lines in 7 files lost coverage.
• Overall coverage increased (+0.03%) to 86.173%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/parser.rs	4	5	80.0%

Files with Coverage Reduction	New Missed Lines	%
tests/sqlparser_common.rs	1	97.32%
tests/sqlparser_bigquery.rs	8	96.24%
src/ast/value.rs	10	85.88%
src/ast/query.rs	31	85.84%
src/tokenizer.rs	91	89.25%
src/ast/mod.rs	272	78.14%
src/parser.rs	414	83.21%

Totals
Change from base Build 4216963554:	0.03%
Covered Lines:	13493
Relevant Lines:	15658

---

💛 - Coveralls

[ankrgyl]

@AugustoFKL @alamb this change looks pretty good to me minus the couple small comments I left.

[alamb]

Thank you @michael-2956 -- I agree with @ankrgyl 's comments. After they are done I think this PR is ready to go.

[alamb]

Thanks @michael-2956