feat: shortcuts configuration

[yjl9903]

Fixes #209

Add shortcuts configuration.

Create ./setup/shortcuts.ts with the following content:

import { defineShortcutsSetup, NavOperations } from '@slidev/types'

export default defineShortcutsSetup((nav: NavOperations) => {
  return [
    {
      key: 'enter',
      fn: () => nav.next(),
      autoRepeat: true,
    },
    {
      key: 'backspace',
      fn: () => nav.prev(),
      autoRepeat: true,
    },
  ]
})

Related type declaration:

export interface NavOperations {
  next: () => void
  prev: () => Promise<void>
  nextSlide: () => void
  prevSlide: () => Promise<void>
  downloadPDF: () => Promise<void>
  toggleDark: () => void
  toggleOverview: () => void
  escapeOverview: () => void
  showGotoDialog: () => void
}

export interface ShortcutOptions {
  key: string
  fn: () => void
  autoRepeat?: boolean
}

export type ShortcutsSetup = (nav: NavOperations) => Array<ShortcutOptions>

export function defineShortcutsSetup(fn: ShortcutsSetup) {
  return fn
}

[antfu]

Could you also open another PR to the docs repo adding these info? Thanks!