TSQL: Fix SET TRANSACTION ISOLATION LEVEL parsing

[jpers36]

Brief summary of the change made

SQLFluff's TSQL dialect does not parse SET TRANSACTION ISOLATION LEVEL statements correctly. This PR will fix it.

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

None.

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).

[codecov]

Codecov Report

Merging #2701 (359e4dd) into main (244c3d5) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##              main     #2701   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files          163       163           
  Lines        12060     12060           
=========================================
  Hits         12060     12060           

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

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