feat: configurable keyboard shortcuts via keyboardShortcuts option

[dpecos]

Summary

• Adds a keyboardShortcuts option to createSlideshow() for remapping, extending, or disabling any key binding
• Rewrites Keyboard class to use KeyboardEvent.key + keydown (replaces the deprecated keyCode / keypress approach)
• 19 new tests covering defaults, overrides, disabling, arrays of keys, and activate/deactivate

API

mdeck.createSlideshow({
  keyboardShortcuts: {
    // Replace default keys for an action
    gotoNextSlide: ['ArrowRight', 'ArrowDown', 'j', ' '],
    // Single key as a string
    togglePresenterMode: 'p',
    // Disable a default binding entirely
    toggleFullscreen: null,
    // Add a completely new action binding
    customAction: 'x',
  }
});

Behaviour

Config value	Effect
string	Replaces default keys for that action
string[]	Replaces default keys with the array
null	Disables that action entirely
omitted	Default keys are kept unchanged

Implementation notes

• DEFAULT_SHORTCUTS is exported so callers can inspect or extend it
• All key matching uses KeyboardEvent.key (e.g. 'ArrowRight', 'j', ' ')
• Single-character keys are normalised to lowercase for case-insensitive matching
• Shift+Space → previous slide is preserved as a special case (not in the keymap, so it cannot be accidentally overridden via the config)
• Digit accumulator (0–9 then Enter → gotoSlideNumber) is also a special case outside the keymap

Closes #8