fix: Improve robustness of IFocusableNode uses

[BenHenning]

The basics

• I validated my changes

The details

Resolves

Fixes RaspberryPiFoundation/blockly-keyboard-experimentation#515

Proposed Changes

This adds canBeFocused() checks to all the places that could currently cause problems if a node were to return false.

Reason for Changes

This can't introduce a problem in current Core and, in fact, most of these classes can never return false even through subclasses. However, this adds better robustness and fixes the underlying issue by ensuring that getFocusableElement() isn't called for a node that has indicated it cannot be focused.

Test Coverage

I've manually tested this through the keyboard navigation plugin. However, there are clearly additional tests that would be nice to add both for the traverser and for WorkspaceSvg, both likely as part of resolving #8915.

Documentation

No new documentation changes should be needed here.

Additional Information

This is fixing theoretical issues in Core, but a real issue tracked by the keyboard navigation plugin repository.

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[BenHenning]

Feel free to take a look @rachel-fenichel if you have a free moment--I don't think this PR needs to be specifically reviewed by anyone in particular and I suspect that it's a largely non-controversial change.

[BenHenning]

Self-reviewed changes.

[BenHenning]

Will be affected a bit by #9033 so waiting for that to be merged, first.