vue-test-utils: allow false as a component stub value

Updated types/index.d.ts to allow false to be passed as a component stub value
in mount or shallowMount. `false` in this case means don't use a stub for the
given component.

Author: garyo

Diff for: packages/test-utils/types/index.d.ts

@@ -22,7 +22,7 @@ type Slots = {
  * If it is an array of string, the specified children are replaced by blank components
  */
 type Stubs = {
-  [key: string]: Component | string | true
+  [key: string]: Component | string | true | false
 } | string[]
 
 /**