Add full support for on conflict clause in SQLite

[Enduriel]

Brief summary of the change made

• Adds upsert clause support with on conflict to the sqlite dialect, as described here. 7ee8d7d
• Adds general support for the on conflict to the sqlite dialect clause as described here closing Unable to parse sqlite on conflict clause #3872 2314d7b
• Fixes alter table clauses not working in sqlite closing SQLite statements with ALTER TABLE are not parsed #5732 ef751dd

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

No

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.

I'm not 100% clear on how you guys structure merges into main, I decided to just make a single branch with all my commits as it was easier for me, if you prefer I can split it back up as the commits are pretty clearly separate with the caveat that 2314d7b depends on ef751dd which is why I structured it this way.

[WittierDinosaur]

LGTM

[Enduriel]

Should also close #5733

[github-actions]

Coverage Results ✅

Name    Stmts   Miss  Cover   Missing
-------------------------------------
TOTAL   17771      0   100%

225 files skipped due to complete coverage.

[coveralls]

coverage: 99.985%. remained the same
when pulling f3a7534 on Enduriel:sqlite-more-missing-features
into d649e97 on sqlfluff:main.

[Enduriel]

@WittierDinosaur the auto merge seems to have failed because of some generic coveralls error, maybe you could try again? I've got some more commits I'd like to merge in but they depend on the features in here and I don't want to tack them on at this point since this PR has a nice scope imo.