fix(cli): improve language selection logic in cliInit function (#806)

Fixes #660 

### Description Changes

When running `cdk init`, if the specified command type can only be
executed in a single language, allow the command to be executed without
selecting a language.

---
By submitting this pull request, I confirm that my contribution is made
under the terms of the Apache-2.0 license

Author: ren-yamanashi

packages/aws-cdk/lib/commands/init/init.ts

@@ -52,21 +52,28 @@ export async function cliInit(options: CliInitOptions) {
     await printAvailableTemplates(ioHelper, options.language);
     throw new ToolkitError(`Unknown init template: ${type}`);
   }
-  if (!options.language && template.languages.length === 1) {
-    const language = template.languages[0];
-    await ioHelper.defaults.warn(
-      `No --language was provided, but '${type}' supports only '${language}', so defaulting to --language=${language}`,
+
+  const language = await (async () => {
+    if (options.language) {
+      return options.language;
+    }
+    if (template.languages.length === 1) {
+      const templateLanguage = template.languages[0];
+      await ioHelper.defaults.warn(
+        `No --language was provided, but '${type}' supports only '${templateLanguage}', so defaulting to --language=${templateLanguage}`,
+      );
+      return templateLanguage;
+    }
+    await ioHelper.defaults.info(
+      `Available languages for ${chalk.green(type)}: ${template.languages.map((l) => chalk.blue(l)).join(', ')}`,
     );
-  }
-  if (!options.language) {
-    await ioHelper.defaults.info(`Available languages for ${chalk.green(type)}: ${template.languages.map((l) => chalk.blue(l)).join(', ')}`);
     throw new ToolkitError('No language was selected');
-  }
+  })();
 
   await initializeProject(
     ioHelper,
     template,
-    options.language,
+    language,
     canUseNetwork,
     generateOnly,
     workDir,

packages/aws-cdk/test/commands/init.test.ts

@@ -22,6 +22,18 @@ describe('constructs version', () => {
     expect(await fs.pathExists(path.join(workDir, 'lib'))).toBeTruthy();
   });
 
+  cliTest("when type is 'lib' and language is not specified, it default language to TypeScript", async (workDir) => {
+    await cliInit({
+      ioHelper,
+      type: 'lib',
+      workDir,
+    });
+
+    // Check that tsconfig.json and lib/ got created in the current directory
+    expect(await fs.pathExists(path.join(workDir, 'tsconfig.json'))).toBeTruthy();
+    expect(await fs.pathExists(path.join(workDir, 'lib'))).toBeTruthy();
+  });
+
   cliTest('can override requested version with environment variable', async (workDir) => {
     await cliInit({
       ioHelper,