enhance(macros/cssxref): strip extra content from displayed link name

[yarusome]

Summary

Fixes #8853.
Depends on #8766.

Problem

To refer to CSS functions and descriptors that are subpages using {{CSSXref}}, currently it's necessary to use a rather repetitive invocation like {{CSSXref("color_value/rgb", "rgb()")}}.

Solution

This PR removes the last slash (if any) and any content before it from the displayed link name when {{CSSXref}} is specified with only one parameter.

---

How did you test this change?

Run the following code in the console:

function getDisplayName(arg0, arg1, arg2, pageType) {
  let displayName = (arg1 || arg0.slice(arg0.lastIndexOf("/") + 1));

  if (!arg1) {
    // Append parameter brackets to CSS functions
    if ((pageType === "css-function") && !displayName.endsWith("()")) {
      displayName += "()";
    }
  }
  
  return displayName;
}

const TEST_CASE = [
  ["color_value/lab", "", "", "css-function"],
  ["@page/size", "", "", "css-at-rule-descriptor"],
  ["abs()", "", "", "css-function"],
]

for (const caseInput of TEST_CASE) {
  console.log(getDisplayName(...caseInput));
}

/* Expected output:
 * lab()
 * size
 * abs()
 */

[github-actions]

This pull request has merge conflicts that must be resolved before it can be merged.

[caugner]

LGTU (@argl @fiji-flo @caugner), thanks!