Sandpack Devtools have landed

This is the start of communicating Sandpack releases and changelog more transparently. Sandpack is still in Beta so there might be some unexpected changes happening every now and then. Here we go:

• SandpackCodeEditor - support to load external CodeMirror extensions, which brings a lot of opportunities such as Eslint integration, vim keybindings, autocomplete, etc; #286.
• Icons - export all icons used internally, from now on, it makes it possible for the consumer app to reuse even more parts of the Sandpack; #288.
• Read-only mode - introduces read-only mode by file or for the entire sandbox; #300.
• SandpackReactDevTools - we finally get the React Devtools working properly in the React docs, there were a lot of technical challenges and we worked very close with Brian from the React core team to get this done; link.

Docs

• Eslint guide - introduce an example of how to integrate Eslint on Sandpack; link.

Bugs fixes

• init-mode: avoid unnecessary remount of the code editor component every time the viewport state has changed. Reported by Framer #294.

What's next

• Working on the Console component - many people have requested it, it seems promising, let's look into it.
• SSR implementation - it's done and waiting for a final review and once it's merged I'm going to use React docs as the first candidate to test it.
• Working with @Marco Vincit to redesign some parts of Sandpack that will bring even more possibilities to the layout.