Add prop-type validations for components

[tterb]

Description

Added prop-type validations for DiscussionEmbed, CommentCount, and CommentEmbed components.

Motivation and Context

Prop-type validation offers minimal typechecking that helps to troubleshoot issues and catch potential bugs.

How Has This Been Tested?

Tested using the disqus-react example website.

Screenshots (if appropriate):

Types of changes

• [ ] Bug fix (non-breaking change which fixes an issue)
• [x] Maintenance (package updates and code-quality improvements)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• [x] My code follows the code style of this project.
• [ ] My change requires a change to the documentation.
  • [ ] I have updated the documentation accordingly.
• [x] All new and existing tests passed.

---

This change is