Control language service availability using project capabilities

[drewnoakes]

Currently we use ILanguageServiceHostEnvironment to control whether the language service should be initialized or not.

project-system/src/Microsoft.VisualStudio.ProjectSystem.Managed.VS/ProjectSystem/VS/LanguageServices/LanguageServiceHostEnvironment.cs

Lines 10 to 34 in fa20f2b

	[Export(typeof(ILanguageServiceHostEnvironment))]
	internal sealed class LanguageServiceHostEnvironment : ILanguageServiceHostEnvironment
	{
	private readonly AsyncLazy<bool> _isEnabled;
	[ImportingConstructor]
	public LanguageServiceHostEnvironment(IVsUIService<SVsShell, IVsShell> vsShell, JoinableTaskContext joinableTaskContext)
	{
	_isEnabled = new(
	async () =>
	{
	await joinableTaskContext.Factory.SwitchToMainThreadAsync();
	// If VS is running in command line mode (e.g. "devenv.exe /build my.sln"),
	// the language service host is not enabled.
	return ErrorHandler.Succeeded(vsShell.Value.GetProperty((int)__VSSPROPID.VSSPROPID_IsInCommandLineMode, out object resultObj))
	&& resultObj is bool result
	&& !result; // negate: enabled when not in command line mode
	},
	joinableTaskContext.Factory);
	}
	public Task<bool> IsEnabledAsync(CancellationToken cancellationToken) => _isEnabled.GetValueAsync(cancellationToken);
	}

Today, we use this interface to prevent the language service from doing redundant work during devenv /build.

The current design requires calling code to check this flag. It would be cleaner to control this via project capabilities.