feat(indent)!: Add Org indent_mode command in favor of toggling vim.b.org_indent_mode

[kristijanhusak]

Summary

Previously, toggling virtual indentation on the buffer was done by manually changing value of the buffer variable vim.b.org_indent_mode to true or false respectively.
This PR introduces an subcommand indent_mode to the global Org command which will toggle the virtual indentation on/off. This allows us to simplify and remove some of the code that might become deprecated in the future.

Changes

• Add Org indent_mode global command
• Remove dict watcher on vim.b.org_indent_mode variable
• Remove some old deprecation messages
• Remove plugin/org.vim file since it's no longer needed

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.