Add dev container workspace

Allows the linting and style settings for the frontend to be applied even when you load the main repo as a workspace

(cherry picked from commit d6278fced49b26be975c3a6039b38a94f700864b)

Closes #3428

.devcontainer/Readarr.code-workspace

@@ -0,0 +1,13 @@
+// This file is used to open the backend and frontend in the same workspace, which is necessary as
+// the frontend has vscode settings that are distinct from the backend
+{
+  "folders": [
+    {
+      "path": ".."
+    },
+    {
+      "path": "../frontend"
+    }
+  ],
+  "settings": {}
+}