chore: drop parentNode property

[SethFalco]

The parentNode property was declared legacy and pending removal during v4, but was still used internally in a few places.

This officially drops XastNode#parentNode so that it does not exist in public nor internal APIs.

This may have an impact on consumers who created custom plugins and imported our style/xast helpers like:

• computeOwnStyle
• querySelectorAll
• querySelector
• matches

Previously, these functions had the context of the whole node tree, even if a child node was passed to it. It is no longer has that context by default.

Usage is unaffected in the following conditions:

• The first parameter (node) is a root node (XastRoot).
• The second parameter (selector) only needs to check children of the first parameter, and so selectors that navigate across nested nodes aren't used, such as or >.

In other cases, a helper has been provided called #mapNodesToParents which maps every node in a tree to its parent node.

For example, you can do:

// applies `selector` with context only of the node and its decendants
const nodes = querySelectorAll(node, selector);

// applies `selector` with the context of the entire node tree relative from `node`
const nodes = querySelectorAll(node, selector, mapNodesToParents(root));

// this usage has the same behavior as v3, as `node` is already the entire node tree 
const nodes = querySelectorAll(root, selector);

Changes

• Drops XastNode#parentNode from internal API.
• computeOwnStyle, querySelectorAll, querySelector, and matches accept an additional optional argument which is a map of children to parent nodes.
• In relation to above, a helper function called mapNodesToParents is now also exported, to make it simple to create the optional parameter for the most common usage. By accepting a Map directly instead of the root, it also avoids the need to revisit every node in cases where they've already been visited by the caller.
• Reorganizes css-select Options so that instead of referencing the legacy parentNode, it now receives the map of children to parents.

Improvements

I would prefer to invoke #mapNodesToParents lazily, only if getParent is invoked and parents is undefined. However, that would mean invoking #mapNodesToParents in css-select-adapter, which would introduce a circular dependency.

For now, I'll leave this as-is and in future I will investigate restructure to avoid that issue.