[Core] Convert custom webpack cell-loader into Babel Plugin

[RobertBroersma]

This implements #503 and opens up the possibility for using Cells in Jest!

I've never written a Babel Plugin or done anything with AST's before, so I just wanted to get this out here to gather some feedback before looking into writing tests for it, and also to discuss on where to locate the plugin file!

This also fixes some issues with Cells where they break if you comment out some const exports and supports using function declarations instead of variable function declarations!

[peterp]

I actually think this is perfect, but I also don't have a bunch of experience with ASTs. @mojombo did the last implementations for our eslint, router, and cell plugins.

I've made a few suggestions for comments and variable names that reduce the abstractness for the reader of this code.

[RobertBroersma]

@peterp Thanks for the feedback! It's processed. 👍

I've decided to name the variable what it is; EXPECTED_EXPORTS_FROM_CELL. Let me know if that doesn't make sense.

I also added some tests for the plugin. I noticed tests weren't running on the core package. Is there a reason for that? I configured them to run so I could run my plugin test.

[peterp]

This is amazing work, thank you so much Robert!

[thedavidprice]

@RobertBroersma great work all around here, Robert, from super helpful inline comments to ramping up tests for the core package! 🚀