This PR improves the logic we use to assign font stacks to type tokens. Now every font will get a stack, even if it has no defined stack.

1. First we look for a custom stack and use it if it exists.
2. Then we look to see if the font has an existing stack defined in its metadata, and use that if it exists.
3. If neither of those exist, we use a predefined default stack for that font type.

I created a new map to hold our new default stacks, using the system stack as our sans default, the Georgia stack as our serif default, and the Bitstream Vera Sans Mono stack as our mono default.

Before:

font-family:Foo Sans,;

After:

font-family:Foo Sans, -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto,
  Helvetica, Arial, sans-serif, "Apple Color Emoji", "Segoe UI Emoji",
  "Segoe UI Symbol";

Fixes #4048