Make citar-insert-citation context aware

[bdarcus]

In markdown and latex, citar-insert-citation should ideally work pretty similarly to how org-cite-insert works:

1. if point is on an existing key, prompt to replace it
2. if point is after existing key, prompt to append a new one (or new ones)
3. if point is before an existing key, prompt to push a new one (or new ones)

For LaTeX, also:

4. if prompt is on a command, prompt to replace it

With this, could simplify the keymaps, and maybe remove the interactive citar-insert-keys.

I also am wondering if we can define org-cite-insert, or a wrapper for it, as the org insert-citation function?

See also #381

[bdarcus]

Closed via #426