fix(useFetch): mark isFinished, isFetching readonly (#3616)

Co-authored-by: Anthony Fu <anthonyfu117@hotmail.com>

packages/core/useFetch/index.ts

@@ -1,14 +1,14 @@
 import type { EventHookOn, Fn, MaybeRefOrGetter, Stoppable } from '@vueuse/shared'
 import { containsProp, createEventHook, toRef, toValue, until, useTimeoutFn } from '@vueuse/shared'
 import type { ComputedRef, Ref } from 'vue-demi'
-import { computed, isRef, ref, shallowRef, watch } from 'vue-demi'
+import { computed, isRef, readonly, ref, shallowRef, watch } from 'vue-demi'
 import { defaultWindow } from '../_configurable'
 
 export interface UseFetchReturn<T> {
   /**
    * Indicates if the fetch request has finished
    */
-  isFinished: Ref<boolean>
+  isFinished: Readonly<Ref<boolean>>
 
   /**
    * The statusCode of the HTTP fetch response
@@ -33,7 +33,7 @@ export interface UseFetchReturn<T> {
   /**
    * Indicates if the request is currently being fetched.
    */
-  isFetching: Ref<boolean>
+  isFetching: Readonly<Ref<boolean>>
 
   /**
    * Indicates if the fetch request is able to be aborted
@@ -528,12 +528,12 @@ export function useFetch<T>(url: MaybeRefOrGetter<string>, ...args: any[]): UseF
   )
 
   const shell: UseFetchReturn<T> = {
-    isFinished,
+    isFinished: readonly(isFinished),
+    isFetching: readonly(isFetching),
     statusCode,
     response,
     error,
     data,
-    isFetching,
     canAbort,
     aborted,
     abort,