Allow contributions of custom CSS properties to CSS extension #47775
Description
The motivation is that today, css Language Service would suggest that you have invalid properties.
microsoft/vscode-css-languageservice#68
Although by adopting MDN data we should solve the problem, however if new properties come up CSS LS can still fall behind.
We should provide a config [css/less/scss].validate.acceptedProperties or something similar that stops CSS LS from complaining about "unknown property".