Fix split part negative part number

[dave-connors-3]

resolves #688

Description

Copies standard approach seen in other core adapters for handling negative arguments to part_number in dbt.split_part

https://github.com/dbt-labs/dbt-core/pull/7200

Checklist

• [x] I have read the contributing guide and understand what's expected of me
• [x] I have signed the CLA
• [x] I have run this code in development and it appears to resolve the stated issue
• [ ] This PR includes tests, or tests are not required/relevant for this PR
• [x] I have opened an issue to add/update docs, or docs changes are not required/relevant for this PR
• [x] I have run changie new to create a changelog entry

[dave-connors-3]

@mikealfare forgot to link the associated core PR that adds the necessary test case we are interested in. once that's in we can remove the pointer from the dev_requirements.txt

https://github.com/dbt-labs/dbt-core/pull/7200