Use nvim's builtin dictionary

[UtkarshVerma]

Is your feature request related to a problem? Please describe.
If we enable nvim's built-in spell-checking using set spell, then nvim can save custom words in ~/.config/nvim/spell/en.utf-8.add.

Describe the solution you'd like
A configuration option that would utilize nvim's spell list.

Describe alternatives you've considered
N/A