Introducing the layout zoo

[Josh Bowman-Matthews]

As we make progress on our rewrite of Servo's layout engine
(https://github.com/servo/servo/wiki/Layout-2020), there have been
various times when other developers with experience building layout
engines have asked us questions that start with "how will you handle"
and "have you considered". To avoid forgetting about these helpful
questions as our engine continues to progress, I have created the Servo
layout zoo: https://github.com/servo/layout-zoo/issues, and seeded it
based on reading past meeting notes.

The layout zoo contains issues that describe a particular interesting
edge case of CSS layout. They are tagged according to the main CSS
features that they interact with, and the goal of this work is to make
it easy for contributors to Servo's layout engine to answer the question
"are there edge cases related to the feature that I'm working on that I
should consider?"

My hope is that we will continue to expand the set of issues
proactively, and anytime there is an in-person discussion that contains
a "have you considered" question that we will immediately add it to the
zoo so that knowledge isn't lost.

Cheers,
Josh