Add Material.Avalonia.TreeDataGrid project with TreeDataGrid styles

[klorman]

Summary

This PR adds a basic Material-styled theme for TreeDataGrid in Material.Avalonia.

Motivation

I needed TreeDataGrid to display a hierarchical structure with virtualization, but noticed that Material.Avalonia doesn't currently include any styling for it.

To keep visual consistency, I created a custom style by referencing:

• existing TreeView and DataGrid styles from this theme
• WPF implementation of Material Design in XAML
• and the official Material Design spec for data tables: https://m2.material.io/components/data-tables

Implementation Notes

• The style is currently implemented without a demo.
• For testing or future demonstration, you can use the sample from the official Avalonia TreeDataGrid repo:
  https://github.com/AvaloniaUI/Avalonia.Controls.TreeDataGrid/tree/master/samples/TreeDataGridDemo

Screenshots

Tree inside TreeDataGrid

Table

[SKProCH]

Thanks!