feat: defaul cmd can now be a function that returns a cmd string

[noby-y]

this simple change makes it possible to define custom functions for default cmds. as an example:

default = function()
	local relative_path = ' ' .. vim.fn.expand('%:.')
	local binary_path = ' ' .. vim.fn.expand('%:.:r')

	local commands = {
		python = 'python' .. relative_path,
		c = 'gcc -o' .. binary_path .. relative_path,
	}

	return commands[vim.bo.filetype]
end,

[noby-y]

how did i misspell the pr :(