Split css/syntaxes.json into several files

Currently css/syntaxes.json contains syntaxes for values, selectors and at-rules. The problem is each type of syntax have its own grammar.

Note that the values related to at-rules are also syntaxes, and are even shared among at-rules and properties in some cases.

I talk about syntaxes that use non CSS Values and Units Module grammar, in particular:

• an-plus-b
• feature-type
• feature-value-block
• feature-value-declaration
• general-enclosed
• keyframe-block
• mf-plain
• mf-range
• page-body
• page-margin-box
• page-margin-box-type
• pseudo-page
• var

Suggestion is split css/syntax.json into three dictionaries, e.g. css/syntax-value.json, css/syntax-atrule.json and css/syntax-selector.json. As an alternative, format of css/at-rule.json and css/selectors.json can be change to host syntaxes too. And css/at-rule.json is already contain some syntaxes with description.

(based on #8)

Start by reading css/syntaxes.json, then compare css/at-rule.json and css/selectors.json, which the issue identifies as possible homes for syntax definitions. Determine which organization best preserves the listed grammar categories and shared at-rule/property syntaxes; the work is done when the agreed structure separates or hosts these definitions without losing the existing entries.