refactor(@angular/build): lazy load sourcemap remapping in js transform worker

Previously, `@ampproject/remapping` was statically imported at the top level
of `javascript-transformer-worker.ts`, eagerly pulling in 5 packages
(`@ampproject/remapping` and 4 sub-dependencies) upon worker initialization.

In standard Angular CLI configurations (`ng build` and `ng serve`), vendor
sourcemaps are disabled by default (`thirdPartySourcemaps = false`), causing
`useInputSourcemap` to evaluate to false for all transformed `node_modules`
files. As a result, sourcemap remapping is never executed in typical builds.

Author: clydin

packages/angular/build/src/tools/javascript-transformer/javascript-transformer-worker.ts

@@ -6,7 +6,7 @@
  * found in the LICENSE file at https://angular.dev/license
  */
 
-import remapping, { type DecodedSourceMap, type EncodedSourceMap } from '@ampproject/remapping';
+import type { DecodedSourceMap, EncodedSourceMap } from '@ampproject/remapping';
 import type { PluginItem } from '@babel/core';
 import { createRequire } from 'node:module';
 import { workerData } from 'node:worker_threads';
@@ -52,6 +52,8 @@ let babelLinkerDeps:
     >
   | undefined;
 
+let remapping: typeof import('@ampproject/remapping').default | undefined;
+
 const textDecoder = new TextDecoder();
 const textEncoder = new TextEncoder();
 
@@ -253,6 +255,7 @@ async function transformJavaScriptImpl(
       }
 
       if (remappingChain.length > 0) {
+        remapping ??= (await import('@ampproject/remapping')).default;
         const finalMap = remapping(remappingChain, () => null).toString();
         const base64Map = Buffer.from(finalMap).toString('base64');
         code += `\n//# sourceMappingURL=data:application/json;charset=utf-8;base64,${base64Map}`;