feat: proxy URI environment variable (#4681)

* Add test extension

This will let us test extension-related features (like the proxy URI).

I removed the environment variables in the script because they override
the ones you set yourself. We still set defaults in constants.ts.

* Add changelog entry for VSCODE_PROXY_URI

* Add terminal test for VSCODE_PROXY_URI

* Update VS Code

This adds the VSCODE_PROXY_URI environment variable.

vendor/package.json

@@ -7,6 +7,6 @@
     "postinstall": "./postinstall.sh"
   },
   "devDependencies": {
-    "code-oss-dev": "cdr/vscode#69a6ce45fc5b883aa8a950e10b79fd083eb0a7d7"
+    "code-oss-dev": "cdr/vscode#d4c3c65d5e17a240a95e735a349e311aaf721b60"
   }
 }