feat(geometry): add face_towards to 2D isometries

[wilmerdooley]

This adds a face_towards constructor for 2D isometries (Isometry2 and IsometryMatrix2) that mirrors the existing 3D behavior. The new method builds an isometry from an eye point and a target point, mapping the local x axis to the view direction and the origin to eye, which is useful for orienting objects toward a target in 2D.

A deprecated new_observer_frame alias is also provided on the 2D types for consistency with the 3D naming, and the rustdoc section in isometry.rs is updated to list the new constructor.

Resolves #492

Changes

• Add Isometry2::face_towards and IsometryMatrix2::face_towards in src/geometry/isometry_construction.rs, computing the rotation from atan2(dy, dx) between eye and target.
• Add deprecated new_observer_frame aliases on both 2D isometry types, delegating to face_towards.
• Update the 2D construction section in the Isometry rustdoc to mention face_towards.
• Add a proptest in tests/geometry/isometry.rs covering both 2D types, verifying that the origin maps to eye and the x axis maps to the normalized view direction.