Add various features around empty content handling

[bterlson]

The renderer now bookkeeps whether contexts have content underneath them. This capability is used to provide:

• createContentSlot which returns a component that exposes whether its children have contents.
• List, For, etc. now handle list items that don't have any content properly.

I've also updated the python components to use the content slot.

[github-actions]

All changed packages have been documented.

• ✅ @alloy-js/core
• ✅ @alloy-js/csharp
• ✅ @alloy-js/python

Show changes

@alloy-js/python - feature ✏️

pass is now emitted when function and class bodies have no content (rather than just no children). For example, show can now be used inside a class body.

@alloy-js/csharp - fix ✏️

Empty namespaces now don't render an empty block.

@alloy-js/core - feature ✏️

Add a createContentSlot API which returns a component which can be used to detect whether content is added inside it. This can be used to conditionally render fallback content or properly format empty components.

@alloy-js/core - feature ✏️

List, For, and their wrapper components now handle when an element has no content. Show can now be used to conditionally render an item in a list, for example.

[timotheeguerin]

🎉🎉