Explore inside out Croissant

[pdurbin]

To start, we'll just create a Croissant file by hand.

Assuming all parties like the direction, we'll implement something in code. That said, as this is completely unspecified, I recommend we make the following code changes in the following order first:

• enable the Croissant exporter by default (move code to main repo) #11254
• Put a smaller version of Croissant into the head of HTML pages #12123
• Croissant 1.1 #12014