Rename ProjectReunion -> WindowsAppSDK

[DrusTheAxe]

Update the repository per the name change (only some docs were changed before, now change the rest - DLLs, etc).

Naming rules:

1. "Windows App SDK" -- docs/displayname/comments/etc and if spaces are legal
2. "WindowsAppSDK" -- everything else
3. "WinAppSDK" -- can be used in some exceptional cases if necesary (i.e. length limits)
   So sweep through the repository changing e.g. "Project Reunion" -> "Windows App SDK", "ProjectReunion" -> "WindowsAppSDK" plus case variations e.g. PROJECTREUNION_FOO -> WINDOWSAPPSDK_FOO.

This change applies to filenames as well as content.

NOTE: This only changes this repository. The aggregator pipeline also needs changes to match. That'll be a separate PR to the build system (i.e. not here).

Internal source code references weren't strictly required for this task but identifying the artifacts needing changes and all their related references and machinery is complicated, so it was easier to just do a full pass through the repository and update everything. As a perk we won't have folks asking 10 years from now "This code says Project Reunion. What is that?" :-)

This doesn't change any mention of Reunion in the ADO repository's backend definitions. The build pipelines are all *.yml so were updated, but if the ADO build definition says Reunion it'll need ES changes to the ADO instance.

There are a few references left:

#1 build\Helix\RunTestsInHelix.proj
<HelixSource>pr/reunion/$(BUILD_SOURCEBRANCH)/</HelixSource> references a backend artifact (branch?)
@alexlamtest for follow up after this PR completes

#2 dev\MRTCore\mrt\Microsoft.Windows.ApplicationModel.Resources\src\Microsoft.Windows.ApplicationModel.Resources.vcxproj
#3 dev\MRTCore\mrt\Microsoft.Windows.ApplicationModel.Resources\src\packages.config
'reunion' appears in version namaes for FrameworkUDK e.g.

<package id="Microsoft.FrameworkUdk.amd64fre" version="10.0.21363.1000-210416-1731.co-release-svc-reunion2104" targetFramework="native" />

This should go away when newer FrameworkUDK's are adopted
@axelandrejs @BenJKuhn for follow up

#4 dev\VSIX\Directory.Build.props

<RestoreSources Condition="'$(RestoreSources)'==''">
            https://pkgs.dev.azure.com/microsoft/ProjectReunion/_packaging/Project.Reunion.nuget.internal/nuget/v3/index.json

This will go away when the NuGet feed's name is changed.
@BenJKuhn for follow up

#5 installer\dev\console.cpp
There's a reference to https://aka.ms/ProjectReunionInstaller which should be https://aka.ms/WindowsAppSDKInstaller but that doesn't resolve. Can be updated at a future data once the URL's available.
@dkbennett @zaryaf for follow up

#6 README.md
*Previously known as Project Reunion*. is a historical reference which expected (the only one that should stay, at least for the foreseeable future).

[DrusTheAxe]

/azp run

[azure-pipelines]

Azure Pipelines failed to run 1 pipeline(s).

[DrusTheAxe]

Thanks for doing this Howard!

Regarding this:

#1 build\Helix\RunTestsInHelix.proj
pr/reunion/$(BUILD_SOURCEBRANCH)/ references a backend artifact (branch?)
@alexlamtest

        Alex Lam (APPX)
        FTE for follow up after this PR completes

This can be changed to:
<HelixSource>pr/WindowsAppSDK/$(BUILD_SOURCEBRANCH)/</HelixSource>

But, TBH, I don't think it matters too much as long as it starts with pr/.
If you want I can do this in a follow-up PR.

Best to handle in a follow up PR. Thanks!

[DrusTheAxe]

/azp run

[azure-pipelines]

Azure Pipelines failed to run 1 pipeline(s).

[DrusTheAxe]

/azp run

[azure-pipelines]

Azure Pipelines failed to run 1 pipeline(s).

[DrusTheAxe]

/azp run

[azure-pipelines]

Azure Pipelines failed to run 1 pipeline(s).

[DrusTheAxe]

/azp run

[azure-pipelines]

Azure Pipelines failed to run 1 pipeline(s).

[DrusTheAxe]

/azp run

[azure-pipelines]

Azure Pipelines failed to run 1 pipeline(s).

[DrusTheAxe]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[DrusTheAxe]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

Issues were addressed