Show inherited indexes separately in DataTree repr

[mokashang]

Description

DataTree already distinguishes Coordinates: (defined at this node)
from Inherited coordinates: (inherited from an ancestor), but its
text and HTML reprs lump every index into a single Indexes: section.
That makes it easy to mistake an inherited index for one added at the
current node, and there was a TODO in _datatree_node_repr
calling this out.

Fix: split the indexes the same way coordinates are split.

• Indexes: shows only indexes whose coord names come from
  node._node_indexes.
• Inherited indexes: shows only indexes whose coord names all come
  from inherited_vars(node._indexes). Like Inherited coordinates:,
  it is only rendered when root=True, so the whole-tree repr still
  attributes each inherited index to the ancestor that defines it and
  does not repeat it on every descendant.

Both the text repr (_datatree_node_repr in formatting.py) and
the HTML repr (_datatree_node_sections in formatting_html.py)
use the same split helper, _split_node_indexes, which walks
_get_indexes_dict(node.xindexes) once and buckets each (names, Index) pair by whether every name in the group is inherited. That
preserves the existing grouping behaviour for indexes that span
multiple coordinates (e.g. PandasMultiIndex).

filter_nondefault_indexes is applied independently to each bucket,
so the display_default_indexes option keeps its current meaning.
The HTML displayed_line_count accounting is updated to include the
new section.

Before, for a tree where the child inherits a non-default RangeIndex
from its parent, the child's repr looked like this:

<xarray.DataTree 'child'>
Group: /child
    Dimensions:  (x: 4)
    Inherited coordinates:
      * x        (x) float64 32B 0 1 2 3
    Data variables:
        data     (x) float64 32B 10.0 20.0 30.0 40.0

After:

<xarray.DataTree 'child'>
Group: /child
    Dimensions:  (x: 4)
    Inherited coordinates:
      * x        (x) float64 32B 0 1 2 3
    Data variables:
        data     (x) float64 32B 10.0 20.0 30.0 40.0
    Inherited indexes:
        x        RangeIndex (start=0, stop=4, step=1)

Checklist

• Closes Indexes not displayed in DataTree repr #10578
• Tests added
• User visible changes (including notable bug fixes) are documented in whats-new.rst
• New functions/methods are listed in api.rst

[welcome]

Thank you for opening this pull request! It may take us a few days to respond here, so thank you for being patient.
If you have questions, some answers may be found in our contributing guidelines.