Problem:
`nvim_create_autocmd` is too verbose and its `callback` requires extra
"nesting".
Solution:
Introduce `nvim_on`. Start using it internally. Then we can get a feel
for how it should look before making it public.
Problem:
Similar to clearmatches(), it's always necessary to provide a fallback
that allows the user to do a "global reset" when something goes wrong.
Solution:
vim.img.del(math.huge) clears all images.
Use kitty's d=A command to clear all placements in a single
escape sequence rather than N individual deletes, also freeing stored
image data not referenced by the scrollback buffer.
