T-SQL: Properly parse collation names

[borchero]

Brief summary of the change made

The main branch of SQLFluff currently parses collation names appearing in COLLATE expressions as column identifiers. For example, the following SQL ...

SELECT table1.col COLLATE Latin1_General_CI_AS
FROM table1;

... generates the following errors with dialect tsql:

L:   1 | P:  27 | L014 | Unquoted identifiers must be consistently lower case.                                                                      
L:   1 | P:  27 | L028 | Unqualified reference 'Latin1_General_CI_AS' found in
                       | single table select.
L:   1 | P:  27 | L028 | Unqualified reference 'Latin1_General_CI_AS' found in
                       | single table select which is inconsistent with previous
                       | references.

Instead, we want to parse the collation as dedicated identifier "collation". This PR causes the above SQL query to be linted without errors.

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

Probably not, but I did modify Expression_A_Grammar. Should be checked carefully.

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.

[codecov]

Codecov Report

Merging #3686 (eda7cb4) into main (7cd64c7) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##              main     #3686   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files          176       176           
  Lines        13359     13363    +4     
=========================================
+ Hits         13359     13363    +4     

Impacted Files	Coverage Δ
src/sqlfluff/dialects/dialect_ansi.py	100.00% <ø> (ø)
src/sqlfluff/dialects/dialect_tsql.py	100.00% <100.00%> (ø)

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[greg-finley]

Nice solution.