Update for Eleventy 2.0.0 milestone

[johnhunter]

When Eleventy 2.0.0 release is imminent we need to:

• Verify eleventy-load works with the new version
• Fix and add test mocks if 11ty plugin interface has changed
• Update the 11ty.compatibility field in package.json

Eleventy 2.0.0 milestone
https://github.com/11ty/eleventy/milestone/38