Add a .components convenience getter to artboard

[SamJakob]

Intended as a syntactic sugar for forEachComponent on the Artboard class that also performs type checks and casting. e.g., where each component has a given name and type (so works like component).

This is useful when certain components (of a specific type) are named with the intention of processing them together (for example, where multiple Shapes are named AppThemeColor so they can be conveniently assigned the app's theme color as their fill color).