css.lang: Add support for some newer CSS units / dimensions
css.lang: Add support for some newer CSS units / dimensions

Currently the units 'vh', 'vw', 'vmin', 'vmax', 'ch', 'rem'
are not highlighted with the same color as e.g. '10px'.
This commit updates the 'dimension' context to fix the issue.

https://bugzilla.gnome.org/show_bug.cgi?id=770193

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