Fix CommandForm currentValues binding

[woksin]

Summary

• Use command propertyDescriptors when applying CommandForm currentValues
• Add regression coverage for edit-form prefill, no-field currentValues validation, and currentValues validation without legacy properties getters
• Re-export mobx observer from arc.react.mvvm for child view-model components

Verification

• yarn test -- commands/CommandForm/for_CommandForm/when_initializing_with_current_values.ts commands/CommandForm/for_CommandForm/when_currentValues_changes.ts commands/CommandForm/for_CommandForm/when_initial_values_and_current_values_both_provided.ts commands/CommandForm/for_CommandForm/when_validateOnInit_is_true_with_currentValues.ts commands/CommandForm/for_CommandForm/when_currentValues_are_used_without_fields.ts
• yarn test -- for_withViewModel/when_exporting_observer.ts
• yarn tsc -p Source/JavaScript/Arc.React/tsconfig.json --noEmit
• yarn tsc -p Source/JavaScript/Arc.React.MVVM/tsconfig.json --noEmit
• yarn build (Source/JavaScript/Arc.React)
• yarn build (Source/JavaScript/Arc.React.MVVM)

[github-actions]

NuGet packages for this PR, e.g. Cratis.Arc:
https://github.com/cratis/arc/packages/1655206?version=20.43.5