fix(v-model): mutate original array for v-model multi checkbox (#2663)

Note: this change will break non-deep `watch` on the `v-model` bound array since the array is no longer replaced. This can be considered part of the Array watch changes in v3 as detailed at https://v3.vuejs.org/guide/migration/watch.html

This is unfortunate but unavoidable since the issue that it fixes is more important: `v-model` should definitely work with a non-ref reactive array.

fix #2662

packages/runtime-dom/src/directives/vModel.ts

@@ -111,11 +111,9 @@ export const vModelCheckbox: ModelDirective<HTMLInputElement> = {
         const index = looseIndexOf(modelValue, elementValue)
         const found = index !== -1
         if (checked && !found) {
-          assign(modelValue.concat(elementValue))
+          modelValue.push(elementValue)
         } else if (!checked && found) {
-          const filtered = [...modelValue]
-          filtered.splice(index, 1)
-          assign(filtered)
+          modelValue.splice(index, 1)
         }
       } else if (isSet(modelValue)) {
         if (checked) {