Polars `pivot_longer` implementation

[samukweku]

PR Description

Please describe the changes proposed in the pull request:

• pivot_longer implementation for polars

This PR is related to #1352 .

PR Checklist

Please ensure that you have done the following:

1. [x] PR in from a fork off your branch. Do not PR from <your_username>:dev, but rather from <your_username>:<feature-branch_name>.

2. [x] If you're not on the contributors list, add yourself to AUTHORS.md.

3. [x] Add a line to CHANGELOG.md under the latest version header (i.e. the one that is "on deck") describing the contribution.
   • Do use some discretion here; if there are multiple PRs that are related, keep them in a single line.

Automatic checks

There will be automatic checks run on the PR. These include:

• Building a preview of the docs on Netlify
• Automatically linting the code
• Making sure the code is documented
• Making sure that all tests are passed
• Making sure that code coverage doesn't go down.

Relevant Reviewers

Please tag maintainers to review.

• @ericmjl

[ericmjl]

🚀 Deployed on https://deploy-preview-1355--pyjanitor.netlify.app

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 96.20253% with 6 lines in your changes missing coverage. Please review.

Project coverage is 94.14%. Comparing base (62c57c6) to head (7a69b54). Report is 15 commits behind head on dev.

:exclamation: Current head 7a69b54 differs from pull request most recent head b5ceef3

Please upload reports for the commit b5ceef3 to get more accurate results.

Additional details and impacted files

@@            Coverage Diff             @@
##              dev    #1355      +/-   ##
==========================================
- Coverage   94.48%   94.14%   -0.35%     
==========================================
  Files          80       82       +2     
  Lines        4367     4523     +156     
==========================================
+ Hits         4126     4258     +132     
- Misses        241      265      +24     

[ericmjl]

@samukweku looks like we are getting merge conflicts on this PR. I'm a bit hesitant to edit the files as I'm not 100% sure what the right final state should look like. Could you resolve the conflicts please?

[ericmjl]

LGTM! Thank you, @samukweku, for the hard work you've been putting in here!