[CSS extracts] Handle headings, multi dfns, and <boolean-expr[]> (#1891)

The `css-values-5` spec uses new patterns that were not supported yet:

- Definitions in headings, see #1890. Such definitions were correctly captured
in the dfns extracts but CSS extraction missed them.
- Definitions that define more than one type at once through `data-lt`
attributes, as discussed in w3c/csswg-drafts#12349.
More generally speaking, CSS extraction did not always take the `data-lt`
attribute into account.
- Definition of the `<boolean-expr[]>` notation, see #1878. The regular
expression used to split production rules wasn't expecting spaces in the
left-hand side. Note extraction drops `[ <test> ]` and ends up with
`<boolean-expr>` as type name. That's not necessary ideal, but there's no good
way to represent that type in the extracts. Consumers need dedicated logic to
handle the notation.

The update also simplifies the selectors used to find production rules in a
document to make them more readable, and to avoid processing the production
rules more than once, as discussed in #1878.

Author: tidoust