Add upsert clause support to SQLite dialect

[Enduriel]

Brief summary of the change made

This adds upsert clause support with on conflict to the sqlite dialect, as described here. Makes progress on #3872, but that still needs https://www.sqlite.org/lang_conflict.html to be implemented which requires overriding ansi.ColumnConstraintSegment entirely, which I'm hoping to do in another PR as in comparison this PR is relatively straightforward.

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

No

Pull Request checklist

• [X] 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.
  • [X] .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.

[github-actions[bot]]

Coverage Results ✅

Name    Stmts   Miss  Cover   Missing
-------------------------------------
TOTAL   17749      0   100%

225 files skipped due to complete coverage.

[coveralls]

coverage: 99.985%. remained the same when pulling 7ee8d7d000fa87887705dc68686bfe654cb2d448 on Enduriel:sqlite-missing-features into dd04c83c9e9ae20eb28a635fc0503f2dc002ab56 on sqlfluff:main.

[Enduriel]

Closed in favor of #5888