[CSS extracts] Handle headings, multi dfns, and <boolean-expr[]>

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 designed for spaces in the
left-hand side.

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