Move collection polyfills to Compatibility dir.

And tweak to match existing code there.

src/Avalonia.Base/CollectionPolyfills.cs

@@ -1,33 +0,0 @@
-#if !NET6_0_OR_GREATER
-using System.Collections.Generic;
-using System.Diagnostics.CodeAnalysis;
-
-namespace Avalonia
-{
-    internal static class CollectionPolyfills
-    {
-        public static bool Remove<TKey, TValue>(
-            this Dictionary<TKey, TValue> o,
-            TKey key,
-            [MaybeNullWhen(false)] out TValue value)
-                where TKey : notnull
-        {
-            if (o.TryGetValue(key, out value))
-                return o.Remove(key);
-            return false;
-        }
-
-        public static bool TryAdd<TKey, TValue>(this Dictionary<TKey, TValue> o, TKey key, TValue value)
-            where TKey : notnull
-        {
-            if (!o.ContainsKey(key))
-            {
-                o.Add(key, value);
-                return true;
-            }
-
-            return false;
-        }
-    }
-}
-#endif

src/Avalonia.Base/Compatibility/CollectionCompatibilityExtensions.cs

@@ -0,0 +1,32 @@
+using System.Collections.Generic;
+using System.Diagnostics.CodeAnalysis;
+
+namespace System;
+
+#if !NET6_0_OR_GREATER
+internal static class CollectionCompatibilityExtensions
+{
+    public static bool Remove<TKey, TValue>(
+        this Dictionary<TKey, TValue> o,
+        TKey key,
+        [MaybeNullWhen(false)] out TValue value)
+            where TKey : notnull
+    {
+        if (o.TryGetValue(key, out value))
+            return o.Remove(key);
+        return false;
+    }
+
+    public static bool TryAdd<TKey, TValue>(this Dictionary<TKey, TValue> o, TKey key, TValue value)
+        where TKey : notnull
+    {
+        if (!o.ContainsKey(key))
+        {
+            o.Add(key, value);
+            return true;
+        }
+
+        return false;
+    }
+}
+#endif