Add special characters to system font
The [font bitmap](https://github.com/aduros/wasm4/blob/main/runtimes/web/assets/font.png) has empty space reserved for ASCII characters 0x80 through 0xA0.

We should use that space for something, like icons for WASM-4 controller buttons, or generic sprites like stars/hearts/etc.

Games can then use hex string literals to easily display these:

```c
text("Press \x80 to jump!", 0, 0);
```

If anyone enjoys making pixel art and would like to modify the .png, I can follow up with updating the encoded images in the runtimes!