[cssom] "insert a CSS rule" should throw if given a valid rule *with* declarations

As currently specified, "insert a CSS rule" invokes two parsing algos from Syntax:

1. First, it invokes "parse a rule", which only succeeds if it finds exactly one valid rule in its input, and nothing else.
2. If that fails, it invokes "parse a block's contents", and only pays attention to the declarations returned from the algo. (The algo returns a mixed list of rules and declaration lists.) If there are any valid declarations, it succeeds; otherwise it fails.

This means that content like @media (width: 100px) {...} --foo: bar; will fail the first check (extra stuff after the rule), then successfully pass the second and append the --foo: bar as a CSSNestedDeclaration object, ignoring the completely valid @media rule that preceded it.

(The algo also seems to be written against a slightly older version of the "parse a block's contents" algo, when it returned rules and declarations as separate lists. It's easy enough to parse what it means given the current return value, but it should probably be fixed.)

I think this should be changed to look at the whole return value, and only succeed if it returned exactly one item that is a list of declarations.

This means that (a) including valid rules mixed with declarations will fail, as I think we want, and (b) including invalid rules will even sometimes fail, if they show up between declarations. Like, --one: 1; @invalid; --two: 2; will produce two separate lists of declarations, each containing a single declaration. However, @invalid; --one: 1; won't trigger that, as it just returns a single list of declarations.

We could even adjust the Syntax algo to return errors (or at least the presence of them), so the CSSOM algo can consistently fail when an invalid rule is used.

Start with the CSSOM “insert a CSS rule” algorithm and compare its calls to CSS Syntax’s “parse a rule” and “parse a block’s contents” algorithms. Trace the current mixed return value and verify the cases described in the issue, including valid rules with declarations and separated declaration lists. Done means the specification consistently rejects the stated invalid inputs and no longer relies on the older return-value description.