added maximum subarray sum

[avin-madhu]

Describe your change:

• Added the maximum subarray sum algorithm using Kadane's Algorithm in the array section of data structures.

Checklist:

• I have read CONTRIBUTING.md.
• This pull request is all my own work -- I have not plagiarized.
• I know that pull requests will not be merged if they fail the automated tests.
• This PR only changes one algorithm file. To ease review, I have opened separate PRs for separate algorithms.
• All new Python files are placed inside an existing directory.
• All filenames are in lowercase characters with no spaces or dashes.
• All functions and variable names follow Python naming conventions.
• All function parameters and return values are annotated with Python type hints.
• All functions have doctests that pass the automated testing.
• The new algorithm includes a URL that points to an external explanation (e.g., Wikipedia or similar).
• If this pull request resolves one or more open issues, the description includes the issue number(s) with a closing keyword: "Fixes #ISSUE-NUMBER".

[cclauss]

Closing tests_are_failing PRs to prepare for Hacktoberfest