feat: add accessor for the workspace svg group

[BeksOmega]

The basics

• I branched from develop
• My pull request is against develop
• My code follows the style guide
• I ran npm run format and npm run lint

The details

Resolves

Work on #7157

Proposed Changes

Adds public accessor for svgGroup_.

Reason for Changes

This is necessary for the scroll options plugin to add a wheel listener, and for the multiselect plugin to add a mouse enter listener.

Test Coverage

N/A

Documentation

N/A

Additional Information

I didn't make getBlockCanvas public because I realized the scroll options plugin is only using it to get the transform. I think we should either be able to use the metrics, or just scrollX and scrollY to do that instead.

App Inventor asked that I fix the svgGroup_ issue this quarter so they don't need to worry about the multiselect plugin breaking (in this way) in the future.

[rachel-fenichel]

I didn't make getBlockCanvas public because I realized the scroll options plugin is only using it to get the transform. I think we should either be able to use the metrics, or just scrollX and scrollY to do that instead.

Using metrics might work but be very slow--just something to keep an eye out for.