[ddc] Create runtime debugger API

[annagrin]

Parent: dart-lang/webdev#1949

Dwds currently generates JavaScript code to retrieve debug information such as class metadata and object fields. Dwds uses this information to convert chrome JavaScript objects to dart vm service protocol objects.

This JavaScript code assumes internal knowledge of DDC-created objects which has been fragile in the past and is going to break with the new type system (#48585)

Add runtime API to ddc so the dwds is oblivious to internal DDC implementation details (as much as possible).