Add an option to build cscope.files

[Coconop]

Is your feature request related to a problem? Please describe.
I've seen a couple of issues where users failed to build db because they didn't build the csope.files. I've ran into this issue myself.

Describe the solution you'd like
Add an option to set the commands to build this cscope.files like we have db_build_cmd. It could be files_list_cmd for example.

Describe alternatives you've considered
I've personnaly setup a keymap for this:

vim.keymap.set("n", "<leader>cl", function()
	-- List files with fd
	local cmd = 'fd -t f -e c -e h > cscope.files'
	-- List files with find
	-- local cmd = 'find . -type f \\( -name "*.c" -o -name "*.h" \\) > cscope.files'

	-- Run the command
	vim.fn.system(cmd)

	-- Notify the user
	print("cscope.files generated")
end, { desc = "Build cscope.files list" })