feat: allow user to set the default todo keyword

[lyz-code]

Summary

fix #897

I will add the tests or documentation if you agree on the implementation @kristijanhusak

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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.

[kristijanhusak]

@lyz-code does this option exist in emacs orgmode?

[kristijanhusak]

From what I found, Emacs does not have this option. I don't plan to add any custom functionality that does not exist in Emacs orgmode.
You can make a PR to add few more events that you can hook up into your own configuration and modify the values.

[lyz-code]

That makes sense, I'll try my best to guess how to do that ᕕ( ᐛ )ᕗ. Any guidance is welcome

[lyz-code]

Given the structure of OrgMappings:insert_todo_heading() and OrgMappings:insert_todo_heading_respect_content I feel that it would make sense to add an event in OrgMappings:_insert_heading_from_plain_line just before the function ends and another in OrgMappings:meta_return before the startinsert.

In both it will use the line number and use self.files:get_closest_headline_or_nil to pass the headline to the event so that it can set the desired keyword.

How does that sound @kristijanhusak ?