feat: add support for hashtag feeds

[LuanRT]

Description

Adds support for retrieving hashtag pages. Closes #301.

Usage

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

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

const hashtag = await yt.getHashtag('macmiller');
console.log('#macmiller:', hashtag);

// Apply a filter to the hashtag feed:
const shorts = hashtag.applyFilter('Shorts');
console.log('Shorts:', shorts);

Type of change

• New feature (non-breaking change which adds functionality)

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