Add missing types for prompt

[sorenlouv]

This PR exports PromptOptions so it can be consumed, and adds a few properties to BasePrompt

This is much less ambitious than #261 but might be easier to merge since the scope is much smaller.

[sorenlouv]

This could also be specified separately as:

  export class State {
    type: string
    name: string
    message: string
    header: string
    footer: string
    error: string
    hint: string
    input: string
    cursor: number
    index: number
    lines: number
    tick: number
    prompt: string
    buffer: string
    width: number
    symbols: any
    styles: any
    required: typeof Set
    cancelled: boolean
    submitted: boolean
    loading: boolean | 'choices'
    readonly status: 'pending' | 'cancelled' | 'submitted'
    _choices: types.Choice[]

    clone(): Omit<State, 'clone' | 'buffer'> & { buffer: Buffer }

    color: Function | any
  }

Source: https://github.com/enquirer/enquirer/pull/261/files?file-filters%5B%5D=.ts#diff-b52768974e6bc0faccb7d4b75b162c99R369