Make ExperimentalFeatures overridable through props files (#14501)

* Make ExperimentalFeatures overridable through props files

* Change files

* Update CsWinRT packages.lock

* Place exp. feature overrides in the correct order

Author: JunielKatarn

change/react-native-windows-2d88d8d6-ef31-45db-a6e4-f51aa3ed5ab0.json

@@ -0,0 +1,7 @@
+{
+  "type": "prerelease",
+  "comment": "Make ExperimentalFeatures overridable through props files",
+  "packageName": "react-native-windows",
+  "email": "[email protected]",
+  "dependentChangeType": "patch"
+}

vnext/ExperimentalFeatures.props

@@ -1,19 +1,20 @@
 <Project>
-  <PropertyGroup>
-    <RnwNewArch>false</RnwNewArch>
-    <UseFabric>false</UseFabric>
-    <UseWinUI3>false</UseWinUI3>
-    <ReactExperimentalFeaturesSet>true</ReactExperimentalFeaturesSet>
-  </PropertyGroup>
   <PropertyGroup Condition="'$(SolutionName)'=='ReactWindows-Desktop'">
-    <RnwNewArch>true</RnwNewArch>
-    <UseFabric>true</UseFabric>
-    <UseWinUI3>true</UseWinUI3>
-    <UseExperimentalWinUI3>true</UseExperimentalWinUI3>
+    <RnwNewArch Condition="'$(RnwNewArch)' == ''">true</RnwNewArch>
+    <UseFabric Condition="'$(UseFabric)' == ''">true</UseFabric>
+    <UseWinUI3 Condition="'$(UseWinUI3)' == ''">true</UseWinUI3>
+    <UseExperimentalWinUI3 Condition="'$(UseExperimentalWinUI3)' == ''">true</UseExperimentalWinUI3>
   </PropertyGroup>
   <PropertyGroup Condition="'$(SolutionName)'=='Microsoft.ReactNative.NewArch'">
-    <RnwNewArch>true</RnwNewArch>
-    <UseFabric>true</UseFabric>
-    <UseWinUI3>true</UseWinUI3>
+    <RnwNewArch Condition="'$(RnwNewArch)' == ''">true</RnwNewArch>
+    <UseFabric Condition="'$(UseFabric)' == ''">true</UseFabric>
+    <UseWinUI3 Condition="'$(UseWinUI3)' == ''">true</UseWinUI3>
+  </PropertyGroup>
+  <!-- Declare the unconditional group last so it does not supersede solution-specific assignments -->
+  <PropertyGroup>
+    <RnwNewArch Condition="'$(RnwNewArch)' == ''">false</RnwNewArch>
+    <UseFabric Condition="'$(UseFabric)' == ''">false</UseFabric>
+    <UseWinUI3 Condition="'$(UseWinUI3)' == ''">false</UseWinUI3>
+    <ReactExperimentalFeaturesSet Condition="'$(ReactExperimentalFeaturesSet)' == ''">true</ReactExperimentalFeaturesSet>
   </PropertyGroup>
 </Project>