Add missing CSS properties

[florianbuehler]

Description of the Change

Currently a few (newer) CSS properties are missing in grammars/css.cson:

• accent-color
• ascent-override
• descent-override
• line-gap-override
• size-adjust

Alternate Designs

None

Benefits

Color highlighting in consuming packages (e.g. https://github.com/microsoft/vscode) will work correctly (currently above mentionend CSS properties are not highlighted).

Possible Drawbacks

None

Applicable Issues

The change will resolve issue https://github.com/atom/language-css/issues/189.