Add a built-in tsp init template for TypeSpec Shared Spec Library (without TypeScript) only for generate shared spec library.

[allenjzhang]

Clear and concise description of the problem

Current library template is aimed at full featured library which expected to have decorators and TypeScript. An pure shared spec library could simplify it for the user.

Checklist

• Follow our Code of Conduct
• Read the docs.
• Check that there isn't already an issue that request the same feature to avoid creating a duplicate.