Carryover evaluation: Flexbox

[foolip]

Let's evaluate whether to continue the Flexbox focus area in Interop 2024.

Original proposals:

• https://github.com/web-platform-tests/interop/issues/2 by @chrishtr
• https://github.com/web-platform-tests/interop/issues/242 by @bfgeek

Tests:

• All tests
• Failing tests

We should also consider whether we should add tests for 2024.