Keys and settings

Keys

fn frame(t, dt) {
    if key_pressed("F8") {          // true only on the frame the key went down
        this.visible = !this.visible;
    }
    if key_down("LeftShift") {      // true while the key is held
        // ...
    }
}

Key names are the game's (GLFW's): "A" to "Z", "Number0" to "Number9", "F1" to "F25", "Kp0" to "Kp9" (keypad), "Space", "Enter", "Escape", "Tab", "Backspace", "Up", "Down", "Left", "Right", "LeftShift", "LeftControl", "LeftAlt" (and Right...), "Comma", "Period", "Minus", "Equal", and so on. Keys typed into a text field or the game's console, or caught by a dialog, don't count, so a hotkey doesn't fire while the player types.

Pick keys the game doesn't use, and tell players about them (in a print at init, your menu entry's label, or your mod's description).

Settings

config_set saves a value for your mod; config_get reads it back, with a default for the first run:

fn init() {
    this.visible = config_get("visible", true);
    this.warp = config_get("warp", 1.0);
}

fn menu() {
    if menu_item("My mod", this.visible) {
        this.visible = !this.visible;
        config_set("visible", this.visible);
    }
}

Values can be bools, integers, floats and strings. They are written to <game documents>/oxidizer/<mod id>.toml at most once a second and when the game closes. That file is outside your mod's folder, so updating the mod keeps them.