Allow multiple [TargetType] and [TargetAssembly] attributes (#1343)

Patch-method discovery already reads both with the plural
MetadataHelper.GetAttributes<T>(), and the docs say additional target
attributes can be specified, but the attributes lacked
AllowMultiple = true, so the compiler rejected a second one (CS0579).
Set AllowMultiple = true on both.

BepInEx.Preloader.Core/Patching/Attributes.cs

@@ -82,7 +82,7 @@ public class PatcherPluginInfoAttribute : Attribute
 /// <summary>
 ///     Defines an assembly that a patch method will target.
 /// </summary>
-[AttributeUsage(AttributeTargets.Method)]
+[AttributeUsage(AttributeTargets.Method, AllowMultiple = true)]
 public class TargetAssemblyAttribute : Attribute
 {
     /// <summary>
@@ -108,7 +108,7 @@ public class TargetAssemblyAttribute : Attribute
 /// <summary>
 ///     Defines a type that a patch method will target.
 /// </summary>
-[AttributeUsage(AttributeTargets.Method)]
+[AttributeUsage(AttributeTargets.Method, AllowMultiple = true)]
 public class TargetTypeAttribute : Attribute
 {
     /// <param name="targetAssembly">The short filename of the assembly of which <paramref name="targetType" /> belongs to.</param>