MySQL: Define date part function names

[kzosabe]

Brief summary of the change made

fixes #5854

The above issue was caused because the function was not defined as a date part function. I added TIMESTAMPDIFF and some functions whose signatures contain a unit in official documentation.

https://dev.mysql.com/doc/refman/8.0/en/date-and-time-functions.html

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

None

Pull Request checklist

• [x] 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.

[github-actions[bot]]

Coverage Results ✅

Name    Stmts   Miss  Cover   Missing
-------------------------------------
TOTAL   17743      0   100%

225 files skipped due to complete coverage.

[coveralls]

coverage: 99.985%. remained the same when pulling 83a0c7dd046a4608b50580d541346eafc8c6b95c on kzosabe:mysql-date-part-function-name into 1d4fd8d288e9d19aa544d3d92885b30788234470 on sqlfluff:main.