feat: agenda visual bulk actions

[celsobenedetti]

Summary

This PR introduces support for visual mode bulk actions in Agenda view.
Here I'm adding only these two functionalities:

• Agenda:archive_visual()
• Agenda:toggle_archive_tag_visual()

If the approach here is endorsed, I hope to create a PR for "add tags" feature as well.

Related Issues

Related #1003

Changes

• Added two new visual mappings for agenda:
  • <prefix>$ -> Agenda:archive_visual
  • <prefix>A -> Agenda:toggle_archive_tag_visual
• Introduced 2 internal Agenda methods:
  • Agenda:_get_visual_selection_lines
  • Agenda:_bulk_action

Checklist

I confirm that I have:

• Followed the
  Conventional Commits
  specification (e.g., feat: add new feature, fix: correct bug,
  docs: update documentation).
• My PR title also follows the conventional commits specification.
• Updated relevant documentation, if necessary.
• Thoroughly tested my changes.
• Added tests (if applicable) and verified existing tests pass with
  make test.
• Checked for breaking changes and documented them, if any.