Add org-attach feature

[broken-pen]

Hi, apologies for dropping such a large PR in your lap!

I've been working on-and-off on attachments and it's in a state now where I believe it works as intended and very similarly to the Emacs version. :slightly_smiling_face:

The feature seemed simple., but ended up ballooning because:

1. the Emacs version uses a lot of Emacs machinery that nvim doesn't have (recursive copying, downloads);
2. I ended up having to modify several utils functions (like orgmode.utils.fs.substitute_path) for my use case.

I've split commits up as much as I could to make accepting/rejecting each individual change easier. I've also split the implementation of org-attach into 6-7 commits to make it easier to review them.

To be clear, I don't expect all changes to go into the main repository. Let me know if there are any design decisions that you'd rather not commit to maintaining. :wink:

Some of my more dubitable choices:

1. I wrapped a lot of vim.uv.fs_* functions in a module orgmode.attach.fs. I simply couldn't figure out how to write an async recursive copy any other way.
2. I wrote a few new dialogs in orgmode.attach.ui. Some look weird because they replicate Emacs dialogs (yes_or_no_or_cancel_slow() is the equivalent of yes-or-no-p) and might not necessarily make sense in nvim.
3. The Emacs version wildly interleaves logic and user interaction. Elisp has a lot of macros that make this work, unlike Lua. To keep the code clear, I've put the actual logic into orgmode.attach.core and wrapped it in orgmode.attach. This separation required passing around a few callbacks and effectively writing every function twice. It may be more work than it's worth.

Thanks again for keeping this project going, it's been helping me a lot!