Postgres: Add Support for PostGIS operators

[burhanyasar]

Brief summary of the change made

Fixes #4260

Only by adding regex lexer for postgis operators as suggested in the discussions in the initial PR: #4824

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

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 - much cleaner than my implementation!

[github-actions]

Coverage Results ✅

Name    Stmts   Miss  Cover   Missing
-------------------------------------
TOTAL   17697      0   100%

225 files skipped due to complete coverage.

[coveralls]

coverage: 99.985%. remained the same
when pulling 7d0198a on burhanyasar:issue-4260-lexer-only
into 965d087 on sqlfluff:main.

[burhanyasar]

LGTM - much cleaner than my implementation!

Thanks for the hint to redo it with lexer changes only.