feat(core): node.defaultChild as shortcut to escape hatch

[eladb]

Convenience property which returns the child with id "Resource"
or "Default" (fails if there are both).

This is useful in order to enable escape hatching.

Closes #2290

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Pull Request Checklist

• Testing
  • Unit test added (prefer not to modify an existing test, otherwise, it's probably a breaking change)
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[mergify]

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