Placeholder does not behave like a placeholder

When setting the `placeholder` property (JSX.Element) for the PlainTextPlugin it adds the JSX.Element as element to the DOM after the editor `div`. This is incorrect placeholder behavior as it just another element in the DOM. It would be expected that the placeholder becomes an actual placeholder property for the editor `div` which has property `contentEditable=true`.

Lexical version: 0.9.0

```
}
placeholder={props.placeholder &&

}
ErrorBoundary={LexicalErrorBoundary}
/>
```

## The current behavior
The placeholder is added is element to the DOM after the editor `div`.

## The expected behavior
The placeholder is a property of the editor `div` that sets a placeholder text when the editor is empty like with below CSS.

```
[contentEditable="true"]:empty:not(:focus):before {
content: attr(placeholder)
}
```