set handlebars as default template engine.

[HugoMario]

PR checklist

• [ x] Read the contribution guidelines.
• [ x] 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\.
• [ x] Filed the PR against the correct branch: 3.0.0 branch for changes related to OpenAPI spec 3.0. Default: master.
• [ x] Copied the technical committee to review the pull request if your PR is targeting a particular programming language.

Description of the PR

set "handlebars" as default template engine.