feat: global vars

[kellyjosephprice]

	Fix RM-11562

🧰 Changes

Allows user variables in esm code.

Prior to this, user variables (eg {user.email}) were being replaced with a JSX component (<Variable name="email" />) opaquely. The actual values were provided by the VariableContext. This mimiced how they were handled in pre-mdx versions of the library. This PR undoes that, and provides the user object as a top level import. The following was not possible before, but is possible now:

export const HelloWorld = () => {
  return <div>{user.email}</div>
}

We had talked about implementing it this way, and I somehow ignored/forgot that. Oops.

---

The VariableContext is still being used, because our custom wrapper around code blocks will also insert the user object into the scope of the code block. eg:

```
Why do we support this? {user.email}
```

Also, I've left the variable transformer processor/transform/variable in place, because it's still being used by the editor. When calling mdast, user variables such as {user.email} should show up as a readme-variable node type.

🧬 QA & Testing

• Broken on production.
• Working in this PR app.