Use 'import type' in TypeScript examples (#1656)

Author: skirtles-code

src/guide/typescript/composition-api.md

@@ -164,7 +164,8 @@ year.value = '2020'
 Sometimes we may need to specify complex types for a ref's inner value. We can do that by using the `Ref` type:
 
 ```ts
-import { ref, Ref } from 'vue'
+import { ref } from 'vue'
+import type { Ref } from 'vue'
 
 const year: Ref<string | number> = ref('2020')
 
@@ -269,7 +270,8 @@ function handleChange(event: Event) {
 Provide and inject are usually performed in separate components. To properly type injected values, Vue provides an `InjectionKey` interface, which is a generic type that extends `Symbol`. It can be used to sync the type of the injected value between the provider and the consumer:
 
 ```ts
-import { provide, inject, InjectionKey } from 'vue'
+import { provide, inject } from 'vue'
+import type { InjectionKey } from 'vue'
 
 const key = Symbol() as InjectionKey<string>
 

src/guide/typescript/options-api.md

@@ -35,7 +35,8 @@ However, the runtime `props` options only support using constructor functions as
 To annotate complex props types, we can use the `PropType` utility type:
 
 ```ts
-import { defineComponent, PropType } from 'vue'
+import { defineComponent } from 'vue'
+import type { PropType } from 'vue'
 
 interface Book {
   title: string
@@ -69,7 +70,8 @@ export default defineComponent({
 Because of a [design limitation](https://github.com/microsoft/TypeScript/issues/38845) in TypeScript, you have to be careful when using function values for `validator` and `default` prop options - make sure to use arrow functions:
 
 ```ts
-import { defineComponent, PropType } from 'vue'
+import { defineComponent } from 'vue'
+import type { PropType } from 'vue'
 
 interface Book {
   title: string