feat(ytkids): add kids#getChannel()

[LuanRT]

Description

Should've been added by #291, but unfortunately I completely forgot about it : ).

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

(async () => {
  const yt = await Innertube.create({ cache: new UniversalCache(), generate_session_locally: true });

  const channel = await yt.kids.getChannel('UCpbpfcZfo-hoDAx2m1blFhg');
  console.log(channel);
})();

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