build(aio): render class/interface "descendants" in API docs (#19343)

For classes, the tree of subclasses is rendered, recursively.

For interfaces, the descendants are separated into child interfaces, which
extend the interface, and classes, which implement the interface.

Closes #19306

aio/tools/transforms/angular-base-package/rendering/filterByPropertyValue.js

@@ -0,0 +1,9 @@
+module.exports = function filterBy() {
+  return {
+    name: 'filterByPropertyValue',
+    process: function(list, property, value) {
+      if (!list) return list;
+      return list.filter(item => item[property] === value);
+    }
+  };
+};

aio/tools/transforms/angular-base-package/rendering/filterByPropertyValue.spec.js

@@ -0,0 +1,14 @@
+const factory = require('./filterByPropertyValue');
+
+describe('filterByPropertyValue filter', () => {
+  let filter;
+
+  beforeEach(function() { filter = factory(); });
+
+  it('should be called "filterByPropertyValue"', function() { expect(filter.name).toEqual('filterByPropertyValue'); });
+
+  it('should filter out items that do not match the given property value', function() {
+    expect(filter.process([{ a: 1 }, { a: 2 }, { b: 1 }, { a: 1, b: 2 }, { a: null }, { a: undefined }], 'a', 1))
+        .toEqual([{ a: 1 }, { a: 1, b: 2 }]);
+  });
+});