Ignore CSS variables

[spenceryue]

Otherwise:

camelCaseProperty("--my-css-var") // => "-MyCssVar"

There's a workaround downstream in fela-dom/rehydration/rehydrateRules.js#L60 that also motivates this change.

A similar fix is needed in hyphenateProperty(). (I'd recommend dropping the dependency on hyphenate-style-name, so the fix could be made here.)