feat(createInjectionState): add defaultValue option

packages/shared/createInjectionState/index.md

@@ -131,3 +131,29 @@ const [useProvideCounterStore, useCounterStore] = createInjectionState((initialV
   return { count, double, increment }
 }, { injectionKey: CounterStoreKey })
 ```
+
+## Provide a custom default value
+
+```ts
+// useCounterStore.ts
+import { computed, ref } from 'vue'
+import { createInjectionState } from '@vueuse/core'
+
+// custom defaultValue
+const CounterStoreDefaultValue = undefined
+
+const [useProvideCounterStore, useCounterStore] = createInjectionState((initialValue: number) => {
+  // state
+  const count = ref(initialValue)
+
+  // getters
+  const double = computed(() => count.value * 2)
+
+  // actions
+  function increment() {
+    count.value++
+  }
+
+  return { count, double, increment }
+}, { defaultValue: CounterStoreDefaultValue })
+```

packages/shared/createInjectionState/index.ts

@@ -7,6 +7,10 @@ export interface CreateInjectionStateOptions<Return> {
    * Custom injectionKey for InjectionState
    */
   injectionKey?: string | InjectionKey<Return>
+  /**
+   * Default value for the InjectionState
+   */
+  defaultValue: Return | (() => Return)
 }
 
 /**
@@ -18,13 +22,14 @@ export interface CreateInjectionStateOptions<Return> {
 export function createInjectionState<Arguments extends Array<any>, Return>(
   composable: (...args: Arguments) => Return,
   options?: CreateInjectionStateOptions<Return>,
-): readonly [useProvidingState: (...args: Arguments) => Return, useInjectedState: () => Return | undefined] {
+): readonly [useProvidingState: (...args: Arguments) => Return, useInjectedState: () => Return | (() => Return) | undefined] {
   const key: string | InjectionKey<Return> = options?.injectionKey || Symbol(composable.name || 'InjectionState')
+  const defaultValue = options?.defaultValue
   const useProvidingState = (...args: Arguments) => {
     const state = composable(...args)
     provideLocal(key, state)
     return state
   }
-  const useInjectedState = () => injectLocal(key)
+  const useInjectedState = () => injectLocal(key, defaultValue)
   return [useProvidingState, useInjectedState]
 }