09_palindromes: Replace regex solution with non-regex version

[MaoShizhong]

Because

319 replaced the regex solution for the palindromes exercise with a non-regex solution, which definitely was the right move given the context of the curriculum at that point as well as regular gripes in the community from learners who get hit in the face with a regex brick wall upon seeing the solution. However, this was only reflected in the solutions branch and not in the main branch.

Since then, changes to the main branch's solutions test file have not been reflected in the solutions branch and it seems this branch is no longer in use/being updated; learners are also unlikely to be aware of this branch and will just use the solutions directory from the main branch.

This PR

• Replaces the main branch 09_palindromes regex-based solution with the non-regex solution from the solutions branch.
• Adapts that solution to the current test suite, which accounts for unevenly spaced numbers which the original did not pass.
• Cleaned formatting of comments and improved their wording for clarity.

Issue

N/A

Additional Information

Proof that the new solution passes all current tests in the main branch:


Pull Request Requirements

• [x] I have thoroughly read and understand The Odin Project Contributing Guide
• [x] The title of this PR follows the location of change: brief description of change format, e.g. 01_helloWorld: Update test cases
• [x] The Because section summarizes the reason for this PR
• [x] The This PR section has a bullet point list describing the changes in this PR
• [ ] If this PR addresses an open issue, it is linked in the Issue section
• [x] If this PR includes any changes that affect the solution of an exercise, I've also updated the solution in the /solutions folder