Improvements on at-rule syntax description

There are some syntax description issues in css/at-rule.json:

• @charset \"<charset>\"; is quite incorrect, should be @charset <string> ;
• @counter-style has a description of block, but declaration refers to wrong types (i.e. system: <counter-system>; but system key in descriptors)
• @font-face had a problem with wrong reference like in @counter-style syntax, but I fixed it in #221
• <group-rule-body> is wrong for the block of @viewport (should be a list of descriptors)

I think we need to change the description of @counter-style, @font-face and @viewport, since those at-rules have a limited list of descriptors and block syntax can be generated from a descriptors section. The rest at-rule descriptions should be revisited too.
I can bring a PR later if there is no blockers for changes (like MDN pages).

Start in css/at-rule.json and inspect the syntax descriptions for @charset, @counter-style, @font-face, and @viewport, comparing each block with its descriptors section. Check the related correction in issue #221 for context. Done means the named at-rules use the correct syntax references and the remaining at-rule descriptions have been revisited.