fix: Change EntityInHierarchy.parentId to string

[engfragui]

While working on https://github.com/Doist/integrations-backlog/issues/418 (upgrade Todoist-Outlook to API v2), I've noticed that some of the errors and test failures looks something like:

      Type '{ ...; }' is not assignable to type 'EntityInHierarchyWithIndent'.
        Type '{ ...; }' is not assignable to type 'Partial<EntityInHierarchy>'.
          Types of property 'parentId' are incompatible.
            Type 'string | null | undefined' is not assignable to type 'number | undefined'.
              Type 'null' is not assignable to type 'number | undefined'.

Since the API v2 now treats all ids as strings (i.e. projectIds, sectionIds, taskIds, etc.), it makes sense to change this EntityInHierarchy.parentId to string as well.

This is apparently only used in Todoist-Outlook (https://github.com/search?l=TypeScript&q=org%3ADoist+EntityInHierarchy&type=Code) so this seems like a safe change.