Add Github workflow to check for entries starting with "/".

[righettod]

Note: After a very useful review by @ItsIgnacioPortal , I changed the approach by creating a dedicated script to perform the check and raise the warning. The GitHub workflow now only call the script. It will make the maintenance and debugging easier to perform.

Hi,

This PR add a GitHub workflow validating the added or modified files for entries starting with a /, focusing on ones with .txt extensions as the dictionaries in the repository were often ending with such extensions.

⚠ The workflow does not modify any files, if entries are found then a warning message is displayed causing the file to be annotated in the build.

👀 The objective is to let the person in charge of reviewing the PR decide if she/he accept the presence of the /.

Example:

💡 This proposal is in reference to the following proposal by @ItsIgnacioPortal in the PR #749 :

https://github.com/danielmiessler/SecLists/pull/749#issuecomment-1155772656

Thank a lot in advance 😃