Use remark-codesandbox for the code examples

[CompuIves]

In this PR I have replaced some code examples with remark-codesandbox. In the repo we specify some template projects, which we then can refer to in our code blocks. For example:

This code will take the umd template (defined in /codesandbox/examples/umd), and replace the entry file with this contents, and then create an embed with those contents included. Like this:

We can specify as many templates as we want. For example, right now I've defined umd (using script tags), simple (single file) and cra (CRA).

I've also included some examples with line highlights! 😄

I'm curious what you think, I think another improvement would be to let this plugin replace the code blocks with something created with react-sandpack. The advantages of using this are:

1. They're "just" React components, so you can customize visuals/behaviour as much as you want
2. Faster performance, as there's no iframe to load for the editor
3. Self-hosted, the bundler can be self-hosted here and will allow you to apply versioning to the bundler + if CSB ever would go down, the examples will stay up

I can work on a second version where we use react-sandpack if you're interested!

[rickhanlonii]

This is so cool

[rickhanlonii]

Do we know what will happen when people browse embedded codesandboxes on unsupported browsers?

[CompuIves]

Hmm, yes this is a good point. Most probably it will just be a console error, but I can catch this maybe and show a better message. By the way, which browsers do you aim to support with the runnable examples? I'll make sure they'll work in those browsers.

[rickhanlonii]

That's a good question, these supported browser options seem fair to me but I defer to @gaearon.

[bvaughn]

Nice work @CompuIves !