[css-properties-values-api] Apply hook limits use of custom property value on the same element

[gregwhitworth]

This was in the spec initially as a problem so I'm converting it to an issue so I can pull it out of the spec.

The apply model means that you can't have two custom properties that apply to the same used values coexisting on the same element. This is especially important to point out since our expectation is for these APIs to be utilized by libs and authors may not be aware of the conflicts.

registerProperty({
  name: '--layout',
  syntax: '<string>',
  inherit: false,
  initialValue: 'none',
  apply: function(used, value) {
    if (value == 'grid') {
        used.width = '500px';
    }
  }
});
registerProperty({
  name: '--extraBorder',
  syntax: '<length>',
  inherit: false,
  initialValue: '0px',
  apply: function(used, value) {
    used.width += value;
  }
});