fix(tooling): container user's uid does not match host's uid (#20082)

Docker `jsii/superchain` image, which is used as a base image of devcontainer, has a
default non-root user `superchain` which UID is `1001`.

If the container host's workspace directory is not owned by UID `1001`, users may not be
able to modify files inside the container because of the insufficient permission.

.devcontainer.json has a workaround option [`updateRemoteUserUID`](https://code.visualstudio.com/docs/remote/devcontainerjson-reference#_general-devcontainerjson-properties)
which updates container's non-root user's UID at runtime.

This PR sets `remoteUser` explicitly in .devcontainer.json to make `updateRemoteUserUID` work.


fixes #19979 

----

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.devcontainer.json

@@ -4,5 +4,6 @@
     "postCreateCommand": "yarn build --skip-test --no-bail --skip-prereqs --skip-compat",
     "extensions": [
         "dbaeumer.vscode-eslint@2.1.5"
-    ]
+    ],
+    "remoteUser": "superchain"
 }