feat: expose store_link to agenda

[thuyen]

Summary

As title, this PR exposes store_link to the Agenda API, so one can create links to headlines while in the agenda view.

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.
• Thoroughly tested my changes.
• Checked for breaking changes and documented them, if any.

[kristijanhusak]

Emacs exposes this through the global function as described here https://orgmode.org/manual/Handling-Links.html#Handling-Links-1.

Since we now have global Org command, I added a :Org store_link command that works in both agenda and org files.

[thuyen]

But without remote_edit we cannot add link to an un-opened file from the agenda right? store_link_to_headline would hit error with get_valid_bufnr since there's no buffer for the file.

Probably we need to make store_link_to_headline works even when the file is not opened.

[kristijanhusak]

Yes, I forgot about ID part. Pushed a fix for that, should be working fine now.