Add dotnet/skills marketplace and enable plugins (#13582)

Add `.github/copilot/settings.json` to configure Copilot agent plugin
marketplaces and enable dotnet plugins:

- `dotnet/arcade-skills` marketplace with `dotnet-dnceng` plugin
- `dotnet/skills` marketplace with `dotnet`, `dotnet-msbuild`,
`dotnet-nuget`, and `dotnet-test` plugins

Relates to (and should replace a big part of) #13574

Author: Evangelink

.github/copilot/settings.json

@@ -0,0 +1,16 @@
+{
+  "extraKnownMarketplaces": {
+    "dotnet-arcade-skills": {
+      "source": {
+        "source": "github",
+        "repo": "dotnet/arcade-skills"
+      }
+    },
+    "dotnet-skills": {
+      "source": {
+        "source": "github",
+        "repo": "dotnet/skills"
+      }
+    }
+  }
+}