feat(dev): update dev server plugin env variable to be an array of pl…

[VitaliyR]

…ugins

Summary

Update env var NETLIFY_INCLUDE_DEV_SERVER_PLUGIN to accept array of plugins instead of one.

---

For us to review and ship your PR efficiently, please perform the following steps:

• Open a bug/issue before writing your code 🧑‍💻. This ensures we can discuss the changes and get feedback from everyone that should be involved. If you`re fixing a typo or something that`s on fire 🔥 (e.g. incident related), you can skip this step.
• Read the contribution guidelines 📖. This ensures your code follows our style guide and
  passes our tests.
• Update or add tests (if any source code was changed or added) 🧪
• Update or add documentation (if features were changed or added) 📝
• Make sure the status checks below are successful ✅

A picture of a cute animal (not mandatory, but encouraged)

[davbree]

how are we verifying that @netlify/plugin-2 plugin is also included?

[VitaliyR]

If some plugin is not there, the tests are failing