fix(ApplicationCommand): return string equivalent of ApplicationCommandOptionType

[JKLorenzo]

Please describe the changes this PR makes and why it should be merged:

Fixes the returned ApplicationCommandOption.type of child-options.

The current output of options:

ApplicationCommand.options = [
  {
    name: 'parent',
    description: 'parent description',
    type: 'SUB_COMMAND', // <-- Returns the string equivalent of the ApplicationCommandOptionType ✅
    options: [
      {
        name: 'child',
        description: 'child description',
        type: 6, // <-- Returns the raw API value of the ApplicationCommandOptionType ❌
      }
    ]
  }
]

The proposed output of options:

ApplicationCommand.options = [
  {
    name: 'parent',
    description: 'parent description',
    type: 'SUB_COMMAND',
    options: [
      {
        name: 'child',
        description: 'child description',
        type: 'USER',
      }
    ]
  }
]

Status and versioning classification:

• Code changes have been tested against the Discord API, or there are no code changes
• I know how to update typings and have done so, or typings don't need updating

Note: This is my first time in contributing to the library so please go easy on me :)