Postgres: Support expressions in arrays

[tunetheweb]

Brief summary of the change made

Fixes #2036

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

Not sure why Postgres overrode the ansi definition. It says it was to allow nested arrays, but they are supported with ANSI. Looks like @WittierDinosaur added this in #1722 but I don't think it's necessary.

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 #2599 (652cbb3) into main (4590bff) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##              main     #2599   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files          163       163           
  Lines        11902     11894    -8     
=========================================
- Hits         11902     11894    -8     

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

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