fixes #87061

It doesn't matter whether I'm using Google Chrome, VS Code, Discord, or a Terminal window: any time a text cursor is blinking, it means that the characters I type will show up there.

And this isn't limited to text fields: if I repeatedly press `Tab` to navigate through a website, there's a visual indicator that goes away if I click away from the window, and it comes back if I click or `Alt+Tab` back into it.

<details open>
<summary>Example (Chrome):</summary>

![focus node](https://github.com/flutter/flutter/assets/10457200/bef42cd9-28e5-4214-b071-b7ef56b26609)

</details>

<details open>
<summary>This PR adds the same functionality to Flutter apps:</summary>

![Flutter demo](https://github.com/flutter/flutter/assets/10457200/6eb34c44-5fb0-4b27-aa10-6606a1eb187e)

</details>