Add "module" query parameter to lessons

[SheaBelsky]

Is your feature request related to a problem? Please describe.
In wayfair-archive/awesome-learning-exercises#52, it was discovered that the main field of a package.json can no longer point to a test file. However, we can specify what the CodeSandbox should open to by specifying ?module=/path/to/file in the query parameter.

Describe the solution you'd like
All lesson files should have a main field, which specifies the entry point for the lesson. If this field is specified, the CodeSandbox should automatically open to that file. If it does not, it should open to the README.

Describe alternatives you've considered
The issue with this feature is that it removes knowledge of where to start from the actual lesson, and adding a dependency on the Awesome Learning platform. I'm not crazy about this as a solution because it makes the lessons less standalone and automatic than they were. Perhaps we should consider not opening to the first file at all, and only opening to a README (which we can accomplish through the same main field, see wayfair-archive/awesome-learning-exercises#52). This would encourage students to become more familiar with CodeSandbox to navigate to the file on their own.