Declare peer dependencies for web-component-tester

[nicojs]

The README.md file of web-component-tester currently states the following:

You get a browser-based testing environment, configured out of the box with:

• Mocha as a test framework.
• Chai assertions.
  [...]
• Sinon and sinon-chai to test just your things.

However, sinon, chai and mocha are not dependencies or peerDependencies of web-component-tester itself. You're apparently expected to 'bring-your-own' when using WCT. This is a very brittle construction at best since you're immune to breaking changes announcements. This is the root cause for issue #3394

The best solution would be to include the required plugins and let WCT come with "batteries included". If for some reason you don't want to do that, we should at least declare the manual dependencies as peerDependencies, so that we know what to install and maintain.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[nicojs]

Hello? Any maintainers?

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.