Use neovim's native JSON support if available

Neovim has JSON marshalling support (neovim/neovim#3417), but Maktaba doesn't take advantage of it and instead falls back to the python or vimscript implementation. We should update Maktaba to detect and use Neovim's JSON support.

Hopefully there are no subtle differences between vim's and neovim's behavior. If so, we may need to ensure certain patches are present or work around them.