MySQL: Added SET TRANSACTION parsing

[Xemptuous]

Brief summary of the change made

Added various ways of parsing SET TRANSACTION statements in MySQL according to the docs

Are there any other side effects of this change that we should be aware of?

None that I am aware of.

Pull Request checklist

• Please confirm you have completed any of the necessary steps below.

• Included test cases to demonstrate any code changes, which may be one or more of the following:

  • .yml rule test cases in test/fixtures/rules/std_rule_cases.
  • .sql/.yml parser test cases in test/fixtures/dialects (note YML files can be auto generated with tox -e generate-fixture-yml).
  • Full autofix test cases in test/fixtures/linter/autofix.
  • Other.

• Added appropriate documentation for the change.

• Created GitHub issues for any relevant followup/future enhancements if appropriate.

[WittierDinosaur]

Chaining Delimited and AnySetOf doesn't really work, as Delimited inherits from OneOf - can you remove the AnySetOf here?

[Xemptuous]

My intention was to only allow a max of 2 of these options, as you can't have more than 2 of these options at once. e.g., this won't work:

SET TRANSACTION ISOLATION LEVEL SERIALIZABLE, READ ONLY, READ WRITE;

I didn't see any args in Delimiter() to allow a max # of items.
Is there a better way to do this?

[WittierDinosaur]

Nah, there's no way to get what you're going for here cleanly. We'd need some kind of DelimitedSet operator, which we don't have at the mo

[github-actions]

Coverage Results ✅

Name    Stmts   Miss  Cover   Missing
-------------------------------------
TOTAL   17554      0   100%

225 files skipped due to complete coverage.

[coveralls]

coverage: 99.985%. remained the same
when pulling d74e1df on Xemptuous:mysql/transaction-isolation-level
into 1f651ef on sqlfluff:main.