Codespace Config: project launching

[Arlodotexe]

Overview

This PR aims to configure the repo so we can launch sample apps in a GitHub Codespace as easily as possible.
Closes #15, closes #16, partially addresses #28

Demo from a freshly rebuilt container:

launch-samples-in-codespace-demo.mp4

Changes

• Added devcontainer config - with PowerShell (for future scripting), .NET 6 and other required components for building WASM in VSCode.
• Renamed root solution - AllExperimentsSamples.sln -> Toolkit.Labs.All.sln
• Added extension recommendation - An optional solution explorer that feels more like Visual Studio, instead of browsing raw files.
• Autostart / autoforward ports -- when an Uno app is available on port 5000, a preview window opens automatically.
• Launch configs - for the multi-sample and current single-sample projects. When a new sample project is created, we'll need to amend this file so it can be launched in a codespace
• Add infrastructure for multi-targeting Uno libraries between WinUI 2 and WinUI 3.

Notes

• Uno requires port 5000 to be made public to view the app in a codespace. After much research, I wasn't able to find a way to automatically make this port public. For now, I've added a label to the port to make instructions a bit clearer.
• First build in a Codespace is slow. Subsequent builds are much quicker.
• Direct linking to specific samples may be possible, but it'll require some tinkering and research to nail down the setup
  • Separate folders can have a different devcontainer files. (Configuring separate containers).
  • Workspaces can be scoped to different folders.
  • You can link directly to a workspace in a codespace like this (see link format, clicking won't work anymore).

[Arlodotexe]

The addition of the WinAppSDK has broken this PR. WASM is .NET 5 and WinAppSDK is .NET 6 (Windows specific), so it's attempting to compile both for libraries and failing because a codespace isn't Windows.

[Arlodotexe]

@michael-hawker Ready to merge