Fix the type of excerpt function

[sunnyone]

The "excerpt" option does not expect a function that returns a string but expects a function that mutates a file object itself.
https://github.com/jonschlinkert/gray-matter#optionsexcerpt

[petejohanson]

I hit this just now too working on a next.js project using this.

That being said, I'm really not sure the point of O, and that seems to trip things up more than it helps. Locally, I tried:

  interface GrayMatterOption<
    I extends Input,
  > {
    parser?: () => void
    eval?: boolean
    excerpt?: boolean | ((input: GrayMatterFile<I>, options: GrayMatterOption<I>) => string)
    excerpt_separator?: string
    engines?: {
      [index: string]:
        | ((input: string) => object)
        | { parse: (input: string) => object; stringify?: (data: object) => string }
    }
    language?: string
    delimiters?: string | [string, string]
  }

And this seems to make everything happy. With the semi-recursive O type, the TS compiler seems to trip up. With the change, I can do:

        const { data, content, excerpt } = matter(fileContents, {
            excerpt: (file: matter.GrayMatterFile<string>, opts: matter.GrayMatterOption<string>) => {
                file.excerpt = file.content.split('\n').slice(0, 5).join('\n')
            },
        })

And TS is happy.

Happy to submit an updated PR if folks like that.