.NET Maui "Single Project"

[jonathanpeppers]

Description of Change

Context: https://github.com/xamarin/xamarin-android/blob/fcd9c30164d4ac0e88440357b5c96b9fdebccb39/Documentation/guides/OneDotNetSingleProject.md

We are aiming to get a new project type working that would have a
.csproj such as:

<Project Sdk="Microsoft.NET.Sdk">
  <PropertyGroup>
    <TargetFramework>net6.0-android;net6.0-ios</TargetFramework>
    <OutputType>Exe</OutputType>
    <SingleProject>true</SingleProject>
  </PropertyGroup>
</Project>

Within the project you would have:

• *.cs all shared code
• iOS/* - iOS only stuff, including Info.plist
• Android/* - Android only stuff, including AndroidManifest.xml

I also used some other new settings that results in a smaller
Info.plist and AndroidManifest.xml:

<ApplicationTitle>Single Project</ApplicationTitle>
<ApplicationId>com.microsoft.maui.singleproject</ApplicationId>
<ApplicationVersion>1.0</ApplicationVersion>
<AndroidVersionCode>1</AndroidVersionCode>

The eventual goal being we might be able to completely get rid of
Info.plist and AndroidManifest.xml for simple apps. You would only
need them for edge-case, platform-specific features.

To get this working within a NuGet package, we can include two new
files:

• Xamarin.Forms.SingleProject.props
• Xamarin.Forms.SingleProject.targets

They only enable things conditionally, if $(SingleProject) is set.

I created a new sample with a single Label to demonstrate things
working.

API Changes

None

Platforms Affected

• Core/MSBuild (all platforms)

Behavioral/Visual Changes

A new $(SingleProject) MSBuild property will opt you into a new, magical project type.

Before/After Screenshots

Not applicable

PR Checklist

• Targets the correct branch
• Tests are passing (or failures are unrelated)