map obj as shallowRef() instead of ref()

[kor3k]

https://github.com/studiometa/vue-mapbox-gl/blob/develop/packages/vue-mapbox-gl/components/MapboxMap.vue#L308

const map = ref();
// should rather be
const map = shallowRef();

https://vuejs.org/api/reactivity-advanced.html#shallowref

Unlike ref(), the inner value of a shallow ref is stored and exposed as-is, and will not be made deeply reactive. Only the .value access is reactive.
shallowRef() is typically used for performance optimizations of large data structures, or integration with external state management systems.

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i believe the map object should be wrapped with shallowRef() instead of ref(), as no inner state is being watched, just .value.

[titouanmathis]

#194 has been merged and will be included in the next release, thanks for your contribution @kor3k!