Add Microsoft.Net.Compilers.Toolset reference so the internal source generator loads

[rcdailey]

Problem

On develop (currently 956ba448b), a clean dotnet restore && dotnet build --configuration Debug --no-restore fails with 233 CS1061 errors about missing FastHasFlags, FastIsDefined, AsInt32, and AsUInt32 members on MouseFlags, ViewportSettingsFlags, ViewArrangement, DimAutoStyle, KeyCode, and others. Those members are emitted by EnumExtensionMethodsGenerator in Terminal.Gui.Analyzers.Internal.

The build also prints this warning, which is the actual cause:

CSC : warning CS9057: Analyzer assembly 'Terminal.Gui.Analyzers.Internal.dll' cannot be used
because it references version '5.3.0.0' of the compiler, which is newer than the currently
running version '5.0.0.0'.

The analyzer project is compiled against Microsoft.CodeAnalysis.CSharp 5.3.0 (pinned in Directory.Packages.props), but Terminal.Gui.csproj does not reference Microsoft.Net.Compilers.Toolset, so MSBuild falls back to the Roslyn 5.0.0 host bundled with the .NET SDK (10.0.100 through at least 10.0.104). That host refuses to load analyzers targeting a newer compiler, the generator is silently dropped, and the solution fails to build.

Microsoft.Net.Compilers.Toolset 5.3.0 is already pinned in Directory.Packages.props and already used by Tests/AppTestHelpers.XunitHelpers/AppTestHelpers.XunitHelpers.csproj. It just never got wired into Terminal.Gui.csproj when the generator was introduced in 12eae5e.

Fix

Add the toolset PackageReference alongside the analyzer ProjectReference in Terminal.Gui/Terminal.Gui.csproj:

<PackageReference Include="Microsoft.Net.Compilers.Toolset" PrivateAssets="all" />

This is the officially documented remedy for CS9057 (see the Roslyn wiki's NuGet packages page: "Installing this package overrides the compiler toolset installed in MSBuild"). The package is dev-only, so PrivateAssets="all" keeps it out of consumers of Terminal.Gui.

Verification

After the change, on .NET SDK 10.0.104:

$ dotnet build --configuration Debug --no-restore
...
Build succeeded.
    0 Warning(s)
    0 Error(s)

No more CS9057, generator loads, solution builds clean.

To pull down this PR locally:

git remote add copilot git@github.com:rcdailey/Terminal.Gui.git
git fetch copilot fix/analyzer-compiler-toolset
git checkout copilot/fix/analyzer-compiler-toolset