refactor(@angular-devkit/schematics): avoid for await...of with promise arrays

The upcoming version of typescript Eslint rules will fail the `await-thenable`
rule for cases of for await...of that use promise arrays. This change
removes the usage to avoid lint failures during the version update.
Ref: https://typescript-eslint.io/rules/await-thenable/#async-iteration-for-awaitof-loops

Author: clydin

packages/angular_devkit/schematics/src/rules/base.ts

@@ -35,8 +35,14 @@ export function empty(): Source {
 export function chain(rules: Iterable<Rule> | AsyncIterable<Rule>): Rule {
   return async (initialTree, context) => {
     let intermediateTree: Observable<Tree> | undefined;
-    for await (const rule of rules) {
-      intermediateTree = callRule(rule, intermediateTree ?? initialTree, context);
+    if (Symbol.asyncIterator in rules) {
+      for await (const rule of rules) {
+        intermediateTree = callRule(rule, intermediateTree ?? initialTree, context);
+      }
+    } else {
+      for (const rule of rules) {
+        intermediateTree = callRule(rule, intermediateTree ?? initialTree, context);
+      }
     }
 
     return () => intermediateTree;