1. "RETURNS NULL ON NULL INPUT" option would fail because "INPUT" was not an unreserved keyword
2. function option segment wouldn't allow multiple comma delimited items
3. "AS", before "BEGIN" was not considered optional

Brief summary of the change made

This PR fixes a number of issues with CREATE FUNCTION syntax in TSQL.

The following issues has been resolved:

1. "RETURNS NULL ON NULL INPUT" option would fail because "INPUT" was not an unreserved keyword
2. function option segment wouldn't allow multiple comma delimited items
3. "AS", before "BEGIN" was not considered optional

Fixed in the following ways:

1. "INPUT" added to tsql unreserved keywords list
2. Delimited() added to FunctionOptionSegment
3. OnOf("AS", optional=True) added to CreateFunction

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