Add support to the css `contain` property

[equinusocio]

contain property is marked as unknown property.

Specs:

• https://drafts.csswg.org/css-containment/

Other links:

• https://developer.mozilla.org/en-US/docs/Web/CSS/contain
• https://developers.google.com/web/fundamentals/getting-started/primers/shadowdom#containment

[MhMadHamster]

Hello, this plugin is using https://github.com/Microsoft/vscode-css-languageservice as a provider for rules and descriptions for them, so you probably should ask for it there.

[equinusocio]

Done https://github.com/Microsoft/vscode-css-languageservice/issues/31