fix(types): support typing $el as SVGElement (vuejs#8809)

Author: Kaoru Hagihara

types/test/vue-test.ts

@@ -19,6 +19,7 @@ class Test extends Vue {
   }
 
   // test property reification
+  $el!: HTMLElement | SVGElement;
   $refs!: {
     vue: Vue,
     element: HTMLInputElement,

types/vue.d.ts

@@ -21,7 +21,7 @@ export interface CreateElement {
 }
 
 export interface Vue {
-  readonly $el: HTMLElement;
+  readonly $el: Element;
   readonly $options: ComponentOptions<Vue>;
   readonly $parent: Vue;
   readonly $root: Vue;