feat(core): add undecorated classes migration schematic (#31650)

Introduces a new migration schematic that follows the given
migration plan: https://hackmd.io/@alx/S1XKqMZeS.

First case: The schematic detects decorated directives which
inherit a constructor. The migration ensures that all base
classes until the class with the explicit constructor are
properly decorated with "@Directive()" or "@Component". In
case one of these classes is not decorated, the schematic
adds the abstract "@Directive()" decorator automatically.

Second case: The schematic detects undecorated declarations
and copies the inherited "@Directive()", "@Component" or
"@Pipe" decorator to the undecorated derived class. This
involves non-trivial import rewriting, identifier aliasing
and AOT metadata serializing
(as decorators are not always part of source files)

PR Close #31650

packages/core/schematics/test/helpers.ts

@@ -0,0 +1,28 @@
+/**
+ * @license
+ * Copyright Google Inc. All Rights Reserved.
+ *
+ * Use of this source code is governed by an MIT-style license that can be
+ * found in the LICENSE file at https://angular.io/license
+ */
+
+/**
+ * Template string function that can be used to dedent the resulting
+ * string literal. The smallest common indentation will be omitted.
+ */
+export function dedent(strings: TemplateStringsArray, ...values: any[]) {
+  let joinedString = '';
+  for (let i = 0; i < values.length; i++) {
+    joinedString += `${strings[i]}${values[i]}`;
+  }
+  joinedString += strings[strings.length - 1];
+
+  const matches = joinedString.match(/^[ \t]*(?=\S)/gm);
+  if (matches === null) {
+    return joinedString;
+  }
+
+  const minLineIndent = Math.min(...matches.map(el => el.length));
+  const omitMinIndentRegex = new RegExp(`^[ \\t]{${minLineIndent}}`, 'gm');
+  return minLineIndent > 0 ? joinedString.replace(omitMinIndentRegex, '') : joinedString;
+}