[typescript] allow $ in var name

Created by: jmini

PR checklist

• Read the contribution guidelines.
• Ran the shell script under ./bin/ to update Petstore sample so that CIs can verify the change. (For instance, only need to run ./bin/{LANG}-petstore.sh and ./bin/security/{LANG}-petstore.sh if updating the {LANG} (e.g. php, ruby, python, etc) code generator or {LANG} client's mustache templates). Windows batch files can be found in .\bin\windows\.
• Filed the PR against the correct branch: master, 4.0.x. Default: master.
• Copied the technical committee: Typescript @TiFu (2017/07) @taxpon (2017/07) @sebastianhaas (2017/07) @kenisteward (2017/07) @Vrolijkx (2017/09) @macjohnny (2018/01)

Description of the PR

Fixes #664 (closed), allow $ in var name to support cases like:

    Dummy:
      type: object
      properties:
        ‘$_type':
          type: string
        'type':
          type: string

(details of the change, additional tests that have been done, reference to the issue for tracking, etc)