[two_dimensional_scrollables] Add alignment to TreeView and TableView (#11353)

This PR implements declarative alignment for the content of TableView and TreeView widgets when that content is smaller than the viewport extent. Users can now use the alignment property to center or otherwise position the entire table/tree within the viewport.
Alignment correctly reverts to start for axes that exceed viewport dimensions.

Fixes flutter/flutter#170349 

- TableView
   - Added an alignment property of type AlignmentGeometry (defaults to Alignment.topLeft).
   - Full support for both horizontal and vertical alignment.
   - Supports AlignmentDirectional to correctly handle TextDirection (LTR/RTL).
   - Works with pinned rows and columns, as well as reversed axis directions.

- TreeView
   - Added an alignment property of type AlignmentGeometry (defaults to Alignment.topLeft).
   - Caveat: Currently only supports the vertical component of the alignment. The tree remains
     aligned to the horizontal "start" to maintain consistent indentation logic and avoid layout
     jumps caused by dynamic, lazily-loaded node widths.

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