feat: add config - disable_default_mappings

[MunifTanjim]

Discussion: #265

[cseickel]

I prefer to express toggles in the positive sense, so I would go with either enable_default_mappings or use_default_mappings as the option name.

Between the two, I think use_default_mappings feels the most natural to me in this case. Would you agree?

[cseickel]

The plenary tests might work for me because my packer setup already has those projects cloned to the correct location. If that's not true for you, something like what I do in the github action might work:

git clone https://github.com/MunifTanjim/nui.nvim ~/.local/share/nvim/site/pack/vendor/start/nui.nvim
git clone --depth 1 https://github.com/nvim-lua/plenary.nvim ~/.local/share/nvim/site/pack/vendor/start/plenary.nvim

It's ok though because you did fix the issue.

Thanks for contributing!