This was causing some strange behavior when creating a new zettel from a
selection, as it would change the character under the cursor after
deleting the selection. This was usually a space, but in words that were
followed by punctuation, it would end up replacing the punctuation
character. Instead, we now just insert the text using `i`.

The on_enter function is called when the buffer is loaded
which triggers a refresh. This was causing double backlink
entries because of the async processing

* Fixes zettel enumeration
* Fixes tag enumeration
* Fixes backlink cache parsing
* Fixes some minor UI quirks
* Updates to new folgezettel syntax
* Cleans up new zettel commands a bit

Renames the ftdetect folder to ftplugin and renames neuron.vim to
markdown.vim. This is more correct as the neuron.vim script *did not*
actually set the filetype using `setfiletype` and instead just set up
some autocmds. This allows for markdown plugins to be installed and
functioning as expected while also fully supporting this neuron plugin

The JSON format and command syntax for Neuron will change for V2, which
breaks several parts of this plugin. This commit contains the start of
some changes to fix compatibility with the new format.

This commit is a work-in-progress and not final.

Define neuron_dir so that it is available for nix installs

this reverts commit 53bfaa8e3e.
see https://github.com/fiatjaf/neuron.vim/pull/48#issuecomment-727273461

Following up on https://github.com/fiatjaf/neuron.vim/issues/28#issuecomment-723274398

Tag support

Wrap adding backlink in has_key check.

Remove rib references.

Address caching issues and directories with special characters.

Shift my tag support branch into this repo