fix(@angular/build): enhance Vitest resolution for optimal package loading

Align Vitest's module resolution with the development server to ensure consistent behavior.
This change configures Vitest to prioritize ESM-optimized package entry points by
setting 'es2020', 'module', and 'main' as main fields and 'es2015', 'es2020', and 'module'
as conditions during module resolution. This improves compatibility and performance
by utilizing modern JavaScript module formats where available.

(cherry picked from commit 9b5a452)

Author: clydin

packages/angular/build/src/builders/unit-test/runners/vitest/plugins.ts

@@ -124,6 +124,10 @@ export async function createVitestConfigPlugin(
           noDiscovery: true,
           include: options.optimizeDepsInclude,
         },
+        resolve: {
+          mainFields: ['es2020', 'module', 'main'],
+          conditions: ['es2015', 'es2020', 'module'],
+        },
       };
 
       const { optimizeDeps, resolve } = config;