Add keys.justPressed() and keys.justReleased() handlers

https://discord.com/channels/728582275884908604/728616340239417375/804141588581711943

```
keys {
var canAttack = true
down(Key.SPACE) {
if (canAttack) {
canAttack = false
attack()
}
}
up(Key.SPACE) {
canAttack = true
}
}
```

```
addUpdater {
if (views.keys.justPressed(Key.SPACE)) {
attack()
}
}
```

a new API could be:

```
keys {
justPressed(Key.SPACE) {
attack()
}
justReleased(Key.SPACE) {
idle()
}
}
```

We could also consider preventing the down event from triggering several times.

Start by tracing the existing keys down and up handlers and the views.keys.justPressed usage shown in the issue. Define the desired justPressed and justReleased behavior, including whether down may fire repeatedly, and verify the API across the supported input paths.

Written by the indexing model from the issue text.