feat: Add slot for additional buttons

[PeterDraex]

What kind of change does this pull request introduce?

This PR enables users to add more buttons to the preview area. They will be rendered right before buttons for "Refresh" and "Open in CodeSandbox".

What is the current behavior?

Users can't add custom buttons to preview.

What is the new behavior?

User can add a button in the following way:

<SandpackPreview
    additionalButtons={
      <button
        className="sp-button"
        style={{ padding: 'var(--sp-space-1) var(--sp-space-3)' }}
        onClick={() => window.alert('Bug reported!')}>
        Report bug
      </button>
    }
  />

What steps did you take to test this? This is required before we can merge, make sure to test the flow you've updated.

Manually tested in the documentation and in a personal project.

Checklist

• Documentation
• Tests
• Ready to be merged
  

[codesandbox-ci]

This pull request is automatically built and testable in CodeSandbox.

To see build info of the built libraries, click here or the icon next to each commit SHA.

Latest deployment of this branch, based on commit 4987877:

Sandbox	Source
Sandpack Blog Example 1	Configuration

[danilowoz]

🚀 Thanks a lot @PeterDraex, awesome work here!