Fix reporting of marks on a markableVoid

[SmilinBrian]

Description
Now that markableVoid() allows marks to be applied to inline voids, our users discovered a problem where the marks applied to a currently-selected markable void are not reported properly by Editor.marks()

I have added a few unit tests for the previously un-tested Editor.marks(), including one for markable void that failed before the fix (markable-void-collapsed).

The if (anchor.offset === 0) test in Editor.marks() that checks if the prevPath is in the same block was effectively returning the marks of any Text that preceded the selected markable void.

Checks

• The new code matches the existing patterns and styles.
• The tests pass with yarn test.
• The linter passes with yarn lint. (Fix errors with yarn fix.)
• The relevant examples still work. (Run examples with yarn start.)
• You've added a changeset if changing functionality. (Add one with yarn changeset add.)