Enable networking for WordPress PR viewer

[ironprogrammer]

While #1225 is being deliberated on for Playground as a whole, I think that having network enabled in the WordPress PR viewer would be beneficial for developers and testers.

I've run into situations where using Playground for a PR wasn't an option to test the changes with different themes or plugins that aren't included by default (e.g. block vs classic themes). The additional load time for this feature is a good trade off for the convenience of not needing to fall back on local testing with a typical PR checkout workflow.

What do you think?