make @types/readable-stream a standard dependency

[kyle-johnson]

The builtin types reference @types/readable-stream but don't include it as a dependency (it's currently in devDependencies) meaning consumers of this package must explicitly include @types/readable-stream in their own dependencies for types to resolve properly. Without this fix, tsc gives errors like the following:

  Overload 1 of 7, '(source: Readable, destination: WritableStream | PipelineDestinationIterableFunction<any> | PipelineDestinationPromiseFunction<any, any>, options?: PipelineOptions | undefined): Promise<...> | Promise<...>', gave the following error.
    Argument of type 'BufferListStream' is not assignable to parameter of type 'WritableStream | PipelineDestinationIterableFunction<any> | PipelineDestinationPromiseFunction<any, any>'.
  Overload 2 of 7, '(streams: readonly (WritableStream | ReadableStream | ReadWriteStream)[], options?: PipelineOptions | undefined): Promise<...>', gave the following error.
    Argument of type 'Readable' is not assignable to parameter of type 'readonly (WritableStream | ReadableStream | ReadWriteStream)[]'.
      Type 'Readable' is missing the following properties from type 'readonly (WritableStream | ReadableStream | ReadWriteStream)[]': length, concat, join, slice, and 19 more.
  Overload 3 of 7, '(stream1: ReadableStream, stream2: WritableStream | ReadWriteStream, ...streams: (WritableStream | ReadWriteStream | PipelineOptions)[]): Promise<...>', gave the following error.
    Argument of type 'BufferListStream' is not assignable to parameter of type 'WritableStream | ReadWriteStream'.

Moving type dependencies into dependencies matches the guidance in Typescript docs https://www.typescriptlang.org/docs/handbook/declaration-files/publishing.html#dependencies

Make sure all the declaration packages you depend on are marked appropriately in the "dependencies" section in your package.json. For example, imagine we authored a package that used Browserify and TypeScript. [...] browserify does not bundle its declaration files with its npm packages, so we needed to depend on @types/browserify for its declarations.

[rvagg]

TIL, thanks for the TS handbook link!

[github-actions]

🎉 This PR is included in version 6.0.11 🎉

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