Command API to expose statement groups

We need a command that exposes:

* an array of statement groups
* start and end range of statement group
* what the statement type is for the statement group
* object references in the statement group (or at least the first statement)

No files, tests, or existing command entry points are named. Start by locating the extension's command registration and the code that represents statements and their groups. Done means a command exposes statement-group ranges, statement types, and object references as described.

Written by the indexing model from the issue text.