chore: switch to isolatedModules for all tests (#273)

This should improve build times significantly.

---
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under the terms of the Apache-2.0 license

Author: rix0rrr

.projenrc.ts

@@ -306,6 +306,11 @@ function genericCdkProps(props: GenericProps = {}) {
     authorUrl: 'https://aws.amazon.com',
     authorOrganization: true,
     releasableCommits: pj.ReleasableCommits.featuresAndFixes('.'),
+    tsJestOptions: {
+      transformOptions: {
+        isolatedModules: true,
+      },
+    },
     jestOptions: {
       configFilePath: 'jest.config.json',
       junitReporting: false,