feat(cli): Add javascript for init-templates/sample-app

[engineforce]

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Refer to #398 and #2525

Many JavaScript developers still prefer working with JS files, this pull request added JavaScript support for init-templates/sample-app, utilizing following TypeScript features that enable the full power of TypeScript in JS files:

1. JSDoc support in JavaScript
2. Type Checking JavaScript Files

Pull Request Checklist

• Testing
  • Unit test added (prefer not to modify an existing test, otherwise, it's probably a breaking change)
  • CLI change?: coordinate update of integration tests with team
  • cdk-init template change?: coordinated update of integration tests with team
• Docs
  • jsdocs: All public APIs documented
  • README: README and/or documentation topic updated
  • Design: For significant features, design document added to design folder
• Title and Description
  • Change type: title prefixed with fix, feat and module name in parens, which will appear in changelog
  • Title: use lower-case and doesn't end with a period
  • Breaking?: last paragraph: "BREAKING CHANGE: <describe what changed + link for details>"
  • Issues: Indicate issues fixed via: "Fixes #xxx" or "Closes #xxx"
• Sensitive Modules (requires 2 PR approvers)
  • IAM Policy Document (in @aws-cdk/aws-iam)
  • EC2 Security Groups and ACLs (in @aws-cdk/aws-ec2)
  • Grant APIs (only if not based on official documentation with a reference)

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

[ozbillwang]

Should you update README as well?

[engineforce]

@ozbillwang Which README? https://github.com/awslabs/aws-cdk/blob/master/README.md already mentioned JavaScript.

[ozbillwang]

@engineforce

No javascript list in language.

cdk init --help

  --language, -l        the language to be used for the new project (default can
                        be configured in ~/.cdk.json)
          [string] [choices: "csharp", "fsharp", "java", "python", "typescript"]