feat: add support for searching within a channel

[LuanRT]

Description

This was already possible before (could be done by simply finding the search tab and then calling its nav endpoint), but I figured it'd make more sense to have a native implementation.

Usage:

import { Innertube, UniversalCache } from 'youtubei.js';

(async () => {
  const yt = await Innertube.create({ cache: new UniversalCache() });
  
  const channel = await yt.getChannel('UCXuqSBlHAE6Xw-yeJA0Tunw');
  const search = await channel.search('e-ink');

  console.log(search.videos);
})();

Type of change

• New feature (non-breaking change which adds functionality)
• This change requires a documentation update

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• I have checked my code and corrected any misspellings