Add #nullable enable on shared files.

Some files in Avalonia.Markup are linked into other projects and their use of `?` was causing warnings in the other projects. Explicitly add `#nullable enable` in these cases for now.

src/Markup/Avalonia.Markup/Markup/Parsers/BindingExpressionGrammar.cs

@@ -6,6 +6,8 @@ using Avalonia.Utilities;
 using System;
 using System.Collections.Generic;
 
+#nullable enable
+
 namespace Avalonia.Markup.Parsers
 {
     internal enum SourceMode

src/Markup/Avalonia.Markup/Markup/Parsers/PropertyPathGrammar.cs

@@ -3,6 +3,8 @@ using System.Collections.Generic;
 using Avalonia.Data.Core;
 using Avalonia.Utilities;
 
+#nullable enable
+
 namespace Avalonia.Markup.Parsers
 {
 #if !BUILDTASK

src/Markup/Avalonia.Markup/Markup/Parsers/SelectorGrammar.cs

@@ -4,6 +4,8 @@ using System.Linq;
 using Avalonia.Data.Core;
 using Avalonia.Utilities;
 
+#nullable enable
+
 // Don't need to override GetHashCode as the ISyntax objects will not be stored in a hash; the 
 // only reason they have overridden Equals methods is for unit testing.
 #pragma warning disable 659