Generate TypeScript decorator signatures from extern dec

[timotheeguerin]

Note this is added to tspd which is not published yet so this can be iterated over without any issues.

Adds a decorator signature generator. Generates 2 files:

• <namespace>.ts : Contains the decorator signatures that can be imported and when declaring the decorators functions
• <namespace>.ts-test.ts: Contains some test using typescript type system to make sure the package does reexport the right $<name> for each decorator

[github-actions]

Changes in this PR will be published to the following url to try(check status of TypeSpec Pull Request Try It pipeline for publish status):
Playground: https://cadlplayground.z22.web.core.windows.net/prs/2122/

Website: https://cadlwebsite.z1.web.core.windows.net/prs/2122/

[azure-sdk]

All changed packages have been documented.

[azure-sdk]

You can try these changes at https://cadlplayground.z22.web.core.windows.net/prs/2122/

Check the website changes at https://tspwebsitepr.z22.web.core.windows.net/prs/2122/

[bterlson]

I did not examine the changes to the decorators in detail, as we discussed these changes offline and I am assuming that all of the changes to types (to bring the typescript-side types inline with the declared signature) are all generally widenings.