Implement `Geometry` API Object

We need to implement that basis for PShape. Let's steal the name from p5 and call this Geometry. Shape is kind of a weird term. Really, this is vertex data and we typically don't refer to 3d models as "shapes."

In Processing4, Shape contains Phong material info in its vertex layout. We definitely don't want to do this because we want to support materials as a first class API.

Steps:

• Make it loadable as a Bevy Mesh.
• Add lifecycle methods.
• Build APIs for interacting with vertex layout, defining vertex attributes, etc.
• Add support for modifying CPU side vertex data.
• Add vertex buffer readback support. This is going to be a bit tricky as Bevy packs multiple meshes into vertex buffer slabs.

このissueではファイルやテストが指定されていません。まず、説明で言及されているProcessing PShapeリファレンスとBevy Mesh統合を確認してください。GeometryがBevy Meshとして読み込まれ、ライフサイクル、vertex-layout、CPU側データ、vertex-buffer readbackのAPIを提供できれば、作業は完了です。