Sandpack meets Dracula, support for custom NPM registries and bug fixes

Features

• Release the Dracula theme 🧛: check out the documentation to know how to use it;
• Introduce custom NPM registries API: from now on, you can self-host a proxy server to provide private packages;
• SandpackCodeEditor: support markdown and plain-text syntax highlight.

Fixes & others:

• Themes: set new error values in the palette in order to improve the contrast between;
• Documentation: add a new Sandpack guide, which introduces live collaboration in the code editor component;
• CodeMirror integration: use the correct method to indent the code on the tab key press;
• SandpackCodeEditor: test-id should not be included in the bundler.