Codespace Config doesn't have proper .NET SDK installed

[michael-hawker]

FYI @Arlodotexe with #145 the SDK was bumped, but this isn't installed by default in the Codespace VM:

Installed SDKs:

Install the [7.0.304] .NET SDK or update [/workspaces/Windows/tooling/global.json] to match an installed SDK.

Learn about SDK resolution:
https://aka.ms/dotnet/sdk-not-found
6.0.411 [/usr/share/dotnet/sdk]
The target process exited without raising a CoreCLR started event. Ensure that the target process is configured to use .NET Core. This may be expected if the target process did not run on .NET Core.

[michael-hawker]

Also, if we do find we need to change the TargetFrameworks as part of #152 to build on WASM now, then we need to change that in the vscode confiig too