Allow optional AS in MERGE statements using SELECT

[tunetheweb]

Brief summary of the change made

Fixes #3270

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

Nope

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]

LGTM

[codecov]

Codecov Report

Merging #3276 (277dbf1) into main (99cc8a5) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##              main     #3276   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files          167       167           
  Lines        12627     12627           
=========================================
  Hits         12627     12627           

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

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