Package Properties tab should expose the new Readme property

[advay26-zz]

Similar to #4905, Embedded Readmes are being introduced and will be included in packages - Embedding and displaying NuGet READMEs.

The behavior for Project Properties would be similar to Icon and License File support:

• Using a file picker, users can select a markdown file on disc (.md)

• On selection, the value updates to the file's relative path

• The project file is updated as below:

    <PropertyGroup>
        ...
        <PackageReadmeFile>readme.md</PackageReadmeFile>
        ...
    </PropertyGroup>

    <ItemGroup>
        ...
        <None Include="..\assets\readme.md" Pack="true" PackagePath=""/>
        ...
    </ItemGroup>

The UI view:

[drewnoakes]

We will likely add support for editor metadata here that sets the FileOpenDialog default extension to .md, as required in the spec.

[drewnoakes]

There is some complexity here, in that the interceptor will need to create/update/remove the None item, in addition to the property itself.

[drewnoakes]

There is some complexity here, in that the interceptor will need to create/update/remove the None item, in addition to the property itself.

That complexity also exists for PackageIcon which is not currently wired up to manage the project item. This works in the legacy pages. We need to handle this.

The mechanism should be similar between PackageIcon and PackageReadmeFile, so we should solve both at the same time. It's likely we can create reuse some code here too. ApplicationIcon (#7167) is likely similar as well.

project-system/src/Microsoft.VisualStudio.ProjectSystem.Managed/ProjectSystem/Rules/PropertyPages/PackagePropertyPage.xaml

Lines 94 to 103 in 034b42b

	<!-- TODO (tomescht):
	We should re-think this. This property is the relative path to the icon file _within the package_; setting it does not guarantee
	that the corresponding file in the project is correctly copied into the package. A better alternative may be to expose a property
	with the path to the file in the project instead, and update this one appropriately when that one changes.
	-->
	<StringProperty Name="PackageIcon"
	DisplayName="Icon file"
	Description="The relative path from the package root to the icon file. In addition to setting this property you need to make sure that the file is included in the package. Image file size is limited to 1 MB. Supported file formats include JPEG and PNG. An image resolution of 128x128 is recommended."
	HelpUrl="https://go.microsoft.com/fwlink/?linkid=2147134"
	Category="General" />

[MiYanni]

I'll be implementing this into the new property pages.