Update css at-rule support for auto-complete

[dstorey]

I’ve not included the at-rules that are not yet supported.

[msftclas]

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[aeschli]

The build failure comes from a hiccup in the build, the pull request is fine.
However, to get the changes in VS Code , the 'generate_browserjs.js' script needs to be run. This will generate browsers.js from the xml file. I'll do that. Thanks @dstorey !

[aeschli]

Note: the @viewport rule will not show up in browsers.js as it is marked browsers="none"