fix: prevent SfSidebar disableBodyScroll triggering on desktop

[sethidden]

As part of vuestorefront/storefront-ui#619
body-scroll-lock was added to SfSidebar.
If you look closely at
https://github.com/vuestorefront/storefront-ui/blob/d5b646a6344e0ab4f5dad214458662af7c8bbd57/packages/vue/src/components/organisms/SfSidebar/SfSidebar.vue#L146

you'll see that disableBodyScroll is called in a watcher on the "visible" prop.
hint: disableBodyScroll() just sets "overflow: hidden" on the HTML
document body, which makes the body unscrollable.
In this case that I'm fixing, it goes like:

The value passed to CategoryFilters's isVisible comes from
useUiState's isFilterSidebarOpen. This value is false by default.

Clicking the "clear" or "apply" button in the filters emits the
"close" event from CategoryFilters - if you look at category.vue that
just calls "toggleFilterSidebar". This will set isVisible to TRUE

Changing isVisible to true triggers SfSidebar's watcher, even though the SfSidebar
it is not currently visible (it should be visible only on mobile)

This in turn runs disableBodyLock() and prevents scrolling

[bartoszherba]

@sethidden As long as it works.... :)