[Slider] Add feature flag to activate flow counterparts

[vursen]

FeatureFlags.SLIDER_COMPONENT

See how it's implemented in MasterDetail for reference

https://github.com/vaadin/flow-components/blob/main/vaadin-master-detail-layout-flow-parent/vaadin-master-detail-layout-flow/src/main/java/com/vaadin/flow/component/masterdetaillayout/MasterDetailLayout.java#L170-L171