Support "ms-app" URI schema to activate applications

# Proposal: Support "ms-app" URI schema to activate applications

## Summary
Add support for a new "ms-app:" URI schema to support launching an app via URI, equivalent to how one can do it via [ActivateApplication()](https://docs.microsoft.com/windows/win32/api/shobjidl_core/nf-shobjidl_core-iapplicationactivationmanager-activateapplication).

The new "ms-app" URI scheme should support many of the options supported by [Windows.System.Launcher.LaunchUriAsync()](https://docs.microsoft.com/uwp/api/windows.system.launcher.launchuriasync?view=winrt-19041), e.g. LauncherOptions.FallbackUri.

## Rationale
Many mechanisms support executing a URI - ShellExecute(), Win-R, TaskManager File/Run, CMD.EXE command line, etc. But there's no support for launching an application via URI, with all the power and functionality of ActivateApplication

shellappsfolder: is a partial workaround but faces severe functional limitations. It's also not available on all Windows platforms; ms-app: should be.

Applications can declare their own custom protocol but ShellExecute("somescheme://foo/bar") has no way to common way to express AO_NOSPLASHSCREEN, LauncherOptions.FallbackUri, .DisplayApplicationPicker, TeatAsUntrusted or other options available through ActivateApplication and LaunchUriAsync.

## Scope
| Capability | Priority |
| :---------- | :------- |
| Support a new protocol for app activation (suggestion: "ms-app") | Must |
| Support all parameters available via ActivateApplication | Must |
| Support applicable options available via LauncherOptions | Should |

## Important Notes
Potential URI design for ms-app scheme:

```
ms-app://appid?options
```

where

* appid = Application User Model ID i.e. the application to activate
* options = ActivateApplication.options and LauncherOptions express as query parameters e.g. `ms-app://somepkgname_1234567890abc!App?designmode&splashscreen=0&errorui=0&TreatAsUntrusted=1`

## Open Questions
What LauncherOptions are important for an ms-app: protocol to support? e.g. FallbackUri

No implementation file or test is identified. Start by reviewing ActivateApplication(), Windows.System.Launcher.LaunchUriAsync(), and the proposed ms-app://appid?options shape, then determine the required activation and LauncherOptions behavior; done means the protocol scope and supported parameters are defined and implemented with coverage for the agreed cases.

Written by the indexing model from the issue text.