scan cmd should be a function which can be evaluated depending on cwd

[Kaiser-Yang]

by default, the scan cmd will list hidden files. in most cases, i do not want to list hidden files except in my dotfiles.

by now i have to evaluate the command before setting up. but when the dir changes, the configuration will not change. to make it work as expected, i have to create auto command for dirchanged then call config setup to change the scan cmd, which is a little bit complicated. therefore i hope the option can be a function, and should be evaluated every time the picker is opened.

[Kaiser-Yang]

Actually, the workspace_scan_cmd should inherit from find_files, because they are both file pickers.