Update css-schema.xml to support CSS Grid

[lannonbr]

Fixed microsoft/vscode#13595.

[msftclas]

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[msftclas]

@lannonbr, thanks for signing the contribution license agreement. We will now validate the agreement and then the pull request.

Thanks, Microsoft Pull Request Bot

[aeschli]

Thanks @lannonbr !
As mentioned there is the danger that these changes get overwritten once we connect to the latest data sources. This is not planned for comming next months tough.

[karocksjoelee]

I am using the latest version of vs code , does this updated ??
cause I am still getting [css] unknown property warning