feat: visual promote/demote headings

[celsobenedetti]

Summary

This implements visual mappings for > and < to demote/promote headings.

It uses a "dumb" string parsing approach:

1. If current selection does not include headline, fallback to vim default behavior.
2. else, interate over each line in visual selection
3. check if line is heading with ^%*+
4. if yes promote/demote heading
5. else, leave unchanged

Related Issues

Closes #860

Changes

• Added OrgMappings:do_promote_visual and OrgMappings:do_demote_visual
• Added visual mappings for > and <

output.mp4

caveat: the demo above is done with org_adapt_indentation = false. The indentation is virtual, with :Org indent_mode<CR>. Only heading lines are modified. If we want to indent all lines in selection this will require some extra logic, based on the select indent_mode. I'm incline to KISS, and only modify the heading lines.

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.