Only share Playground testing details when appropriate

[desrosj]

Follow up to #59416.

When a PR is opened to wordpress-develop, a comment is left detailing how to test the changes in a Playground instance. But this is only necessary when there are changes to the built source of WordPress. For example, changes only to the tests/phpunit directory should not require testing in Playground.

Here's an initial list of files that could potentially result in changes that could benefit from Playground testing:

• The src directory
• The (package|package-lock).json files
• Gruntfile.js
• tools/webpack
• webpack.config.js

Limiting when the bot comments will help limit noise on PRs and avoid confusion about how to test certain changes.