Pinia 
by Pig Fang @g-plane
Create a type-level function whose types is similar to [Pinia](https://github.com/posva/pinia) library. You don't need to implement function actually, just adding types.
### Overview
This function receive only one parameter whose type is an object. The object contains 4 properties:
- `id` - just a string (required)
- `state` - a function which will return an object as store's state (required)
- `getters` - an object with methods which is similar to Vue's computed values or Vuex's getters, and details are below (optional)
- `actions` - an object with methods which can do side effects and mutate state, and details are below (optional)
### Getters
When you define a store like this:
```typescript
const store = defineStore({
// ...other required fields
getters: {
getSomething() {
return 'xxx'
}
}
})
```
And you should use it like this:
```typescript
store.getSomething
```
instead of:
```typescript
store.getSomething() // error
```
Additionally, getters can access state and/or other getters via `this`, but state is read-only.
### Actions
When you define a store like this:
```typescript
const store = defineStore({
// ...other required fields
actions: {
doSideEffect() {
this.xxx = 'xxx'
return 'ok'
}
}
})
```
Using it is just to call it:
```typescript
const returnValue = store.doSideEffect()
```
Actions can return any value or return nothing, and it can receive any number of parameters with different types.
Parameters types and return type can't be lost, which means type-checking must be available at call side.
State can be accessed and mutated via `this`. Getters can be accessed via `this` but they're read-only.
