Modifier cleanup needs improvement #149
Description
Currently, the modifier calls floating-ui's autoUpdate in its modify method, which will run whenever an input changes. The method captures the cleanup function that autoUpdate returns and registers it as a destructor, but that can still result in multiple autoUpdates being registered.
Instead, the modifier should capture the last cleanup method and call it at the beginning of the modify method, if present. (There will be none captured the first time the modifier instance runs.)