Add option to configure branch for github_edit_link

[runphp]

The github_edit_link tag currently generates edit links that point to the gh-pages branch by default. This is not ideal when using other default branches like main.
It would be helpful to have an option to configure the target branch for github_edit_link, so it can work properly for repositories using non-gh-pages default branches.

Proposal
Add a configuration option such as github_edit_branch to _config.yml:
yaml
github_edit_branch: main
This will make github_edit_link generate links pointing to that branch.
The user can then set this to main, master or other branches as needed.
This will provide more flexibility when using github_edit_link for repositories with different branch setups.

Benefits

• Works for non-gh-pages default branch
• Easy configuration in _config.yml
• More flexibility for users
• Links point to correct branch for editing

Please consider adding this option in a future release. Let me know if any clarification is needed!