slotProps (type, slot, typeProps) {
    let props = Array.isArray(this.options.slotProps[slot]) ? this.options.slotProps[slot] : []
    const def = this.options.library[type]
    if (def && def.slotProps && Array.isArray(def.slotProps[slot])) {
      props = props.concat(def.slotProps[slot])
    }
    return props.reduce((props, prop) => Object.assign(props, { [prop]: typeProps[prop] }), {})
  }
Given a type and a slot, get the relevant slot props object.
@param {string} type
@param {string} slot
@return {object}