Add useCanvasProvider hook and customTheme option to CanvasProvider to allow users to customize the theme object.

[mannycarrera4]

🚀 Feature Proposal

The CanvasProvider is great but it doesn't allow users to customize the theme to add additional properties like we may not support. Giving users useCanvasProvider hook can allow them to listen to changes on the theme object so that they can style components based on the theme object. An example is density. While we don't support density, consumers should be able to add a density property on the theme object instead of implementing their down context provider and implementation.