Support expression evaluation when running

Expression evaluation in a debugger while running is supported currently on other platforms. Investigate what needs to be done to support it in dwds.

- Dart code allows evaluation in a root library when app is not paused, but dart DevTools does not
- Suggestions:
- call `vmService.evaluate()` on root library ID in DevTools if app is not paused.
- See dart-code:
https://github.com/Dart-Code/Dart-Code/blob/aeea8df97ebe3de1c805f7767bd662cb3bc8f895/src/debug/dart_debug_impl.ts#L1576

- dwds does not always give correct root library
- Add root library to ddc metadata (probably in a new app-level metadata section?)

- Question: is it possible to support general eval while running (ie pick/find a library to evaluate in?)
- Should be possible in DevTools, would need UI changes (to pick the library).
- Is it possible to find the library automatically?

**VSCode debugging on MacOS**

**VSCode debugging on chrome**

Start with the linked dart_debug_impl.ts reference and the vmService.evaluate() suggestion, then trace how dwds determines and exposes the root library through DDC metadata. Determine how DevTools should evaluate while the app is running and whether general evaluation needs library selection. Done means the required scope and behavior are established, including correct root-library handling.

Written by the indexing model from the issue text.