feat: allow enabling safety mode

[LuanRT]

Description

Adds support for safety mode, see #266.
Unrelated: this also simplifies the creation of sessions without a player instance.

Usage:

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

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

  // ...
})();

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