Making the countdown block customizable

Introduction

Now we have an h1 element rendered, it's possible to used it to display information that depend on the component's properties (props). For that, some concepts will be shown, given that they are essential for the app development.

Concepts

Hook

Hooks are APIs that allow using React features inside functional components. Without the hooks, a functional React component is only able to render UI elements. They, hereby, allow, among other things, state storage between different renders and execute side effets to the component's life cycle. Note: They do not work inside classes.

e.g:
```
const [count, setCount] = useState(0)
```
props interface

Defines the props Typescript typing that the component will receive, allowing IDE's intellisense about the created component.
```
interface CountdownProps {
  exampleProp: string
}
```
Block's schema

In VTEX IO, we offer a content management tool called Site Editor. With it, through VTEX Admin, it's possible to change images, texts and behaviours of blocks without having to change the Store's code.

In order for your block to accept user customizations, you need to export a schema in the React component responsible for the block using JSON schema. This will, automatically, generate a form in Site Editor linked to the block that you're developing. Here's a schema example:
```
// react/Countdown.tsx
Countdown.schema = {
    title: 'editor.countdown.title',
    description: 'editor.countdown.description',
    type: 'object',
    properties: {},
}
```
The schema is also responsible for defining the labels that will be displayed to the user when editing the block content on the form.

Atividade

In the interface defined in Countdown.tsx, add a prop called targetDate, its type is string. We are, hence, defining a component prop that will be used to initialize the countdown.

The prop definition itself is made through its declaration in the CountdownProps interface in the Countdown.tsx file, shown previously. Thus, add a line that define the targetDate prop of type string:
```
// react/Countdown.tsx
interface CountdownProps {
+   targetDate: string    
}
```
Now, we need to use it on the component, substituting the text used used previously, Countdown Test, for another, using Site Editor.

Keep in mind that targetDate will be used to define the countdown ending date. However, for now, it will work as a dummy field.

First, change the component in order for it to use the targetDate prop. To do that, you need to use its variable inside the h1 of the React component.
```
// react/Countdown.tsx
const Countdown: StorefrontFunctionComponent<CountdownProps> = ({ targetDate }) => {
  return (
    <div>
      <h1>{ targetDate }</h1>
    </div>
  ) 
}
```

Furthermore, to be able to edit this property through Site Editor, it's necessary to add that same prop to the schema. This is done by adding the targetDate key to the properties object of the schema:

// react/Countdown.tsx
Countdown.schema = {
title: 'editor.countdown.title',
description: 'editor.countdown.description',
type: 'object',
properties: {
+   targetDate: {
+      title: 'Final date',
+      description: 'Final date used in the countdown',
+      type: 'string',
+      default: null,
+   },
},
}

All set! Now you can change the text content through Site Editor. Go ahead to the Site Editor and click on Countdown on the side menu, this will open an edit menu, like the shown bellow:

Now, in the field below the title, type the date in the format AAAA-MM-DD and see the change, that will then show the text you've typed!

brcandido / store-block