perf: Improved performance and reduced memory consumption

[negezor]

Hi. Applying all the same optimizations as here https://github.com/vueform/multiselect/pull/411. In this world, a little optimization never hurt anyone 😅

In addition to the previous PR, the watchers are grouped here to reduce the number of effectScope created. And since we don't need to make all nouislider properties reactive, it's better to leave this job to shallowRef().