accept Uint8Array on dispatcher

[kamecha]

It is useful when the other denops plugin which use Uint8Array use denops.dispatch("sixel_view", "img2sixel", file);.

For example...

const file: Uint8Array = await downloadFile(url);
const sixel = denops.dispatch("sixel_view", "img2sixel", file);
  // ↑ before this PR this line is Error (file is not String)

[gw31415]

Nice PR! Thanks