[css-values-5] Add `resolution` to `<attr-type>`

[SebastianZ]

<attr-type> defines different kinds of dimension keywords, though it is currently missing resolution for resolutions.

Related to that, <resolution> should also be added to the definition of <dimension-unit>. Alternatively, instead of listing all the dimensions, it could just refer to the definition of <dimension>, so there is only one place of truth.

Sebastian

[tabatkins]

Now that attr() is defined with <syntax>, and <resolution> is a valid syntax value, this is handled.