[css-pseudo] Add boundary element and line break element selector.

[JF98]

The following English text comes from machine translation and may not be accurate:

:top-child is used to select the closest child element to the top of the parent container.
:right-child is used to select the closest child element to the right side of the parent container.
:bottom-child is used to select the closest child element to the bottom of the parent container.
:left-child is used to select the closest child element to the left side of the parent container.

:wrap-child selects child elements that are in a wrapped position within the container（"sub-elements arranged in a non-first row）.

If :top-child is implemented, then :wrap-child may not need to be added, but can instead be achieved using the following method:

.parent > :not(:top-child) {
  /* style */
}

We also need you to confirm whether the above suggestions are limited to child elements within the document flow.

[JoshTumath]

Do you have a use case for these suggested pseudo-classes?

[fantasai]

In general we don't add pseudo-classes that depend on layout, since this can be cyclic.