feat: use vim.F.if_nil for set config default value

[fitrh]

See https://github.com/NTBBloodbath/doom-one.nvim/issues/32#issuecomment-1315344450

For config with true as the default value, using or is not the right solution since false or true is true, which means even if the user sets the config to false, the config still run as true.

vim.F.if_nil under the hood compares its value to nil

function if_nil(a, b)
        if a == nil then
                return b
        end
        return a
end

[NTBBloodbath]

Hey thank you so much, very appreciated