Add MustBeVariant attribute and analyzer

- MustBeVariant attribute can be used to enforce that generic types must
be a marshable from/to Variant.
- Also renames all diagnostic ids to be valid unicode identifiers.

modules/mono/glue/GodotSharp/GodotSharp/Core/Array.cs

@@ -483,7 +483,7 @@ namespace Godot.Collections
     /// <typeparam name="T">The type of the array.</typeparam>
     [SuppressMessage("ReSharper", "RedundantExtendsListEntry")]
     [SuppressMessage("Naming", "CA1710", MessageId = "Identifiers should have correct suffix")]
-    public sealed class Array<T> :
+    public sealed class Array<[MustBeVariant] T> :
         IList<T>,
         IReadOnlyList<T>,
         ICollection<T>,

modules/mono/glue/GodotSharp/GodotSharp/Core/Attributes/MustBeVariantAttribute.cs

@@ -0,0 +1,11 @@
+using System;
+
+namespace Godot
+{
+    /// <summary>
+    /// Attribute that restricts generic type parameters to be only types
+    /// that can be marshaled from/to a <see cref="Variant"/>.
+    /// </summary>
+    [AttributeUsage(AttributeTargets.GenericParameter)]
+    public class MustBeVariantAttribute : Attribute { }
+}

modules/mono/glue/GodotSharp/GodotSharp/Core/Dictionary.cs

@@ -352,7 +352,7 @@ namespace Godot.Collections
     /// </summary>
     /// <typeparam name="TKey">The type of the dictionary's keys.</typeparam>
     /// <typeparam name="TValue">The type of the dictionary's values.</typeparam>
-    public class Dictionary<TKey, TValue> :
+    public class Dictionary<[MustBeVariant] TKey, [MustBeVariant] TValue> :
         IDictionary<TKey, TValue>,
         IReadOnlyDictionary<TKey, TValue>
     {

modules/mono/glue/GodotSharp/GodotSharp/GodotSharp.csproj

@@ -56,6 +56,7 @@
     <Compile Include="Core\Attributes\ExportCategoryAttribute.cs" />
     <Compile Include="Core\Attributes\ExportGroupAttribute.cs" />
     <Compile Include="Core\Attributes\ExportSubgroupAttribute.cs" />
+    <Compile Include="Core\Attributes\MustBeVariantAttribute.cs" />
     <Compile Include="Core\Attributes\RPCAttribute.cs" />
     <Compile Include="Core\Attributes\ScriptPathAttribute.cs" />
     <Compile Include="Core\Attributes\SignalAttribute.cs" />