Provide accessors for methods relevant to the scroll options plugin

[BeksOmega]

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Problem

The scroll options plugin accesses some public + underscore properties of the workspace. We should consider whether to add public accessors for these, since there is obviously a use case for accessing them.

The first property is svgGroup_.

The second property is svgBlockCanvas_. This /is/ currently returned by the accessor getBlockCanvas, but getBlockCanvas is marked @internal, so it is inaccessible.

Request

Consider adding a getSvgGroup method and making the getBlockCanvas method public.

Alternatives considered

N/A

Additional context

RaspberryPiFoundation/blockly-samples#1666

[maribethb]

Let's do it

[gonfunko]

Closing as there is now a public WorkspaceSvg.getSvgGroup(), and, while WorkspaceSvg.getBlockCanvas() is still @internal, it just calls and returns WorkspaceSvg.getCanvas(), which is public.