feat: add group_elements_by_parent_id utility function

[MkDev11]

Add a utility function to group elements by their parent_id metadata field. This allows users to easily traverse document hierarchy by grouping elements that share the same parent.

Includes an optional 'assign_orphans' parameter that, when True, assigns elements with no parent_id to the same group as the previous element.

Fixes #1489

[MkDev11]

@lawrence-u10d @badGarnet could you please take a look at the PR? thanks for your time and feedback.

[MkDev11]

=== Test 1: assign_orphans=False (default) ===
  parent_A: ['Title 1', 'Child of A']
  None: ['Orphan 1', 'Orphan 2']
  parent_B: ['Title 2']

=== Test 2: assign_orphans=True ===
  parent_A: ['Title 1', 'Child of A', 'Orphan 1']
  parent_B: ['Title 2', 'Orphan 2']

=== Test 3: First element is orphan, assign_orphans=True ===
  None: ['First orphan']
  parent_A: ['Title 1', 'Orphan 1']

=== All tests passed! ===

[badGarnet]

=== Test 1: assign_orphans=False (default) ===
  parent_A: ['Title 1', 'Child of A']
  None: ['Orphan 1', 'Orphan 2']
  parent_B: ['Title 2']

=== Test 2: assign_orphans=True ===
  parent_A: ['Title 1', 'Child of A', 'Orphan 1']
  parent_B: ['Title 2', 'Orphan 2']

=== Test 3: First element is orphan, assign_orphans=True ===
  None: ['First orphan']
  parent_A: ['Title 1', 'Orphan 1']

=== All tests passed! ===

please add those as unit test

[MkDev11]

=== Test 1: assign_orphans=False (default) ===
  parent_A: ['Title 1', 'Child of A']
  None: ['Orphan 1', 'Orphan 2']
  parent_B: ['Title 2']

=== Test 2: assign_orphans=True ===
  parent_A: ['Title 1', 'Child of A', 'Orphan 1']
  parent_B: ['Title 2', 'Orphan 2']

=== Test 3: First element is orphan, assign_orphans=True ===
  None: ['First orphan']
  parent_A: ['Title 1', 'Orphan 1']

=== All tests passed! ===

please add those as unit test

added it

[MkDev11]

@badGarnet could you please have a look at the changes and let me know ther result? always appeciate for your time!