Add support for release and rollback to savepoint syntax

[CDThomas]

This PR adds support for:

• RELEASE SAVEPOINT savepoint_name statements
• TO SAVEPOINT savepoint_name in ROLLBACK statements.

Note that support for SAVEPOINT savepoint_name statements was already added in #438, so I didn't need to include that here.

Both ROLLBACK ... TO SAVEPOINT savepoint_name and RELEASE SAVEPOINT savepoint_name are part of the SQL grammar (see here and here). At least Postgres (see here and here), SQLite (see here), and MySQL (see here) support savepoints.

The SAVEPOINT keyword is optional when used in ROLLBACK and RELEASE statements in Postgres and SQLite; required in both statements in the SQL grammar; and optional in ROLLBACK, but required in RELEASE in MySQL. This PR makes the SAVEPOINT keyword optional while parsing, but includes it in the canonical format used when printing since all dialects that I've checked so far at least allow for including it.

[coveralls]

Pull Request Test Coverage Report for Build 6935596781

• 37 of 37 (100.0%) changed or added relevant lines in 3 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage increased (+0.02%) to 87.707%

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Files with Coverage Reduction	New Missed Lines	%
src/ast/mod.rs	1	79.01%

Totals
Change from base Build 6935126371:	0.02%
Covered Lines:	17894
Relevant Lines:	20402

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💛 - Coveralls

[alamb]

Looks great to me -- thank you @CDThomas and @tobyhede

[alamb]

I believe the docs CI test failure is real: https://github.com/sqlparser-rs/sqlparser-rs/actions/runs/6818722260/job/18864307303?pr=1045

The lint failure should be resolved with a merge up from main

[CDThomas]

Thank you @alamb and @tobyhede. I've merged in main and fixed up the CI issues.

[alamb]

Thanks again @CDThomas