Code Quality: Migrate files from packaging project to main `Files.App` project

[Lamparter]

Description

What about starting from merging Files (Package) folder contents into Files.App folder?
Since folder name and project name doesn't match and that way enables us to remove WAP easily later.

This proposal is to move files from the packaging project (Files.Package) to the main project (Files.App), including:

• Package.appxmanifest
• Assets/AppTiles/*

While the packaging project can't be fully removed, it would be excellent if these files were moved into the main project and then referenced by the packaging project, like this:

<!-- Inside Files.Package.wapproj -->
<ItemGroup>
  <Content Include="$(RootDirectory)\src\Files.App\Package.appxmanifest">
    <CopyToOutputDirectory>Always</CopyToOutputDirectory>
  </Content>
</ItemGroup>

Concerned code

Contents of Files.Package:

• Package.appxmanifest
• Assets/AppTiles/*

Gains

• Easier overview of package files
• Allows for gradual removal of WAP project

Requirements

• Move files to Files.App
• Reference back in Files.Package.wapproj

Comments

Succeeds #16350

[Lamparter]

@0x5bfa you discussed this with @yaira2 a few months ago.
What do you think?

[0x5bfa]

When we do this, we may as well think about doing single project packaging, utilizing a script/gha to bundle and package to msixupload from x64 and arm64 msix and msixsym.

@yaira2 what was your idea around this, even I make a very simple script you'd not like it?