feat: reactivate horspool

[vil02]

Description of Change

There is some commented out code in horspool.go.
This PR reactivates it by making all necessary changes.

When the given pattern is not present in the input string, I used a similar approach to the one in ./search.

Checklist

• Added description of change
• Added tests and example, test must pass
• PR title follows semantic commit guidelines
• Search previous suggestions before making a new one, as yours may be a duplicate.
• I acknowledge that all my contributions will be made under the project's license.

Notes:
Reactivates horspool.go.

[raklaptudirm]

Add some inline comments to the algorithm implementation explaining it as far as possible.

[vil02]

@raklaptudirm I have added some comments and restructured the code a bit. Now it should be more readable.