exp/textinput: textinput should work even without the IME implementation

[hajimehoshi]

Operating System

• [ ] Windows
• [ ] macOS
• [X] Linux
• [X] FreeBSD
• [X] OpenBSD
• [X] Android
• [X] iOS
• [X] Nintendo Switch
• [X] PlayStation 5
• [X] Xbox
• [ ] Web Browsers

What feature would you like to be added?

Now textinput.Field does nothing for Linux, but we can have a pseudo implementation with the current Ebitengine API, even though IME would not work there.

Why is this needed?

This would improve user experience with textinput