[feat] Bundle for AppSync resolver mapping templates

[vicary]

This is a Feature Proposal

Description

For feature proposals:

• What is the use case that should be solved. The more detail you describe this in the easier it is to understand for us.

The AppSync team is planning for JavaScript support in resolver mapping template https://github.com/aws/aws-appsync-community/issues/147, supporting TypeScript via webpack should be the next trivial thing to do.

• If there is additional config how would it look

No new config should be necessary, but aside from the normal entries, it should also tap into the hooks of serverless-appsync-plugin for resolver mapping templates and append to webpack entries if the configured runtime is javascript.