CSS: Support recursive calc definition, improve compatibility for custom properties, support :has pseudo-class

[matthiasblaesing]

Support recursive CSS property definitions (calc function with embedded function calls)

The grammar for calc property values failed to parse embedded var and
calc calls correctly. The issue is the work-around introduces as
function-content, which basicly prevents nesting any expression that
contains braces.

This changes the grammar parsing result from a tree to a directed graph,
which can contain cycles. This needs to taken into account by useers of
the parsed grammar.

An example of such a cycle is:

calc-fn -> calc-sum -> calc-product -> calc-value -> calc-sum
              ^                                        |
              +----------------------------------------+

For example this is now supported:

.header {
  height: calc(28 * var(--spacing-unit));
}

Improve compatibility for custom properties

Custom properties can hold more or less arbitrary content. This was not parsed and is now:

:root {
  --grid-gutter: (var(--margins) * var(--spacing-unit));
  --cell-margin: (var(--grid-gutter) / 2);
}

Support CSS :has pseudo-class

This was not parsed and is now:

body:has(+ #aCheck:checked) {
    background-color:red;
}

Closes: #5025
Closes: #5303

[vieiro]

Great job! Thanks, @matthiasblaesing

[matthiasblaesing]

@vieiro thank you for the review. I pushed updates - could you have another look?

[vieiro]

@vieiro thank you for the review. I pushed updates - could you have another look?

@matthiasblaesing Great job. Thanks again for this!