fix(@angular-devkit/build-angular): experimental esbuild pipeline, add es2015 to main fields for RxJS v6 compatibility

Currently, for RxJS v6, no conditions are available, and the ESBuild
pipeline will select the ES5 distribution based on the `module` main
field. This is fine in most cases, but applications could benefit from
better optimization with the use of the ES2015 output + there are
certain code differences that currently would cause runtime breakages
when e.g. `rxjs/testing` is used.

See: #25405 for more details.

Fixes #25405.

(cherry picked from commit 9d9a668)

Author: devversion

packages/angular_devkit/build_angular/src/tools/esbuild/application-code-bundle.ts

@@ -47,7 +47,11 @@ export function createCodeBundleOptions(
     assetNames: outputNames.media,
     target,
     supported: getFeatureSupport(target),
-    mainFields: ['es2020', 'browser', 'module', 'main'],
+    // Note: `es2015` is needed for RxJS v6. If not specified, `module` would
+    // match and the ES5 distribution would be bundled and ends up breaking at
+    // runtime with the RxJS testing library.
+    // More details: https://github.com/angular/angular-cli/issues/25405.
+    mainFields: ['es2020', 'es2015', 'browser', 'module', 'main'],
     conditions: ['es2020', 'es2015', 'module'],
     resolveExtensions: ['.ts', '.tsx', '.mjs', '.js'],
     metafile: true,