Fix parsing of Compound Statements

[jonyscathe]

Extra WITH statements could be associated with the compound statement.
Fixed by replacing match_grammar StartsWith and parse_grammar optional
Sequnce with a match_grammar Sequence.

Brief summary of the change made

Fixes #3092

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).
  • 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.

[tunetheweb]

LGTM - assuming tests pass

[codecov]

Codecov Report

Merging #3121 (355a0ea) into main (ddc3200) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##              main     #3121   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files          165       165           
  Lines        12267     12266    -1     
=========================================
- Hits         12267     12266    -1     

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

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