fix(toolkit): ignore hidden files for 'cdk init'

Don't block 'cdk init' on the presence of files start with '.'.

In particular, don't care about the presence of a '.git' directory.

Fixes #1758.

packages/aws-cdk/lib/init.ts

@@ -208,7 +208,7 @@ async function initializeProject(template: InitTemplate, language: string, canUs
 
 async function assertIsEmptyDirectory() {
   const files = await fs.readdir(process.cwd());
-  if (files.length !== 0) {
+  if (files.filter(f => !f.startsWith('.')).length !== 0) {
     throw new Error('`cdk init` cannot be run in a non-empty directory!');
   }
 }