Add layout-instability support

[guillaume-fr]

https://github.com/WICG/layout-instability
https://www.chromestatus.com/feature/5110682739539968

Can be used to compute Cumulative Layout Shift

[Schweinepriester]

Available now via web-features: https://caniuse.com/wf-layout-instability