refactor(devcontainer): reorder properties

Reorder the file so that related properties are roughly grouped together and the order is more logical.

PR: zmkfirmware#476

.devcontainer/devcontainer.json

@@ -1,11 +1,11 @@
 {
   "name": "ZMK Development",
   "dockerFile": "Dockerfile",
-  "extensions": ["ms-vscode.cpptools"],
   "runArgs": ["--security-opt", "label=disable"],
   "containerEnv": { "WORKSPACE_DIR": "${containerWorkspaceFolder}" },
+  "mounts": ["type=volume,source=zmk-config,target=/workspaces/zmk-config"],
+  "extensions": ["ms-vscode.cpptools"],
   "settings": {
     "terminal.integrated.shell.linux": "/bin/bash"
-  },
-  "mounts": ["type=volume,source=zmk-config,target=/workspaces/zmk-config"]
+  }
 }