feat: insert mode promote/demote heading

[celsobenedetti]

Summary

This PR extends the standard vim insert mode <C-d> and <C-t> to promote/demote the heading in current line (:help i_CTRL-D).

behavior:

1. If current line is a heading, promote/demote heading
2. Else, return default <C-d>/<C-t> behavior

rationale
I think this is intuitive UX, and a neat QoL improvement. This is also the behavior in Doom Emacs.

Changes

• Introduced two new functions in mappings.lua: insert_mode_promote and insert_mode_demote.
• Added two new insert mode mappings: <C-d> and <C-t>.

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.