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.

test/e2e/extensions/test-extension/package.json

@@ -0,0 +1,29 @@
+{
+  "name": "code-server-extension",
+  "description": "code-server test extension",
+  "version": "0.0.1",
+  "publisher": "cdr",
+  "activationEvents": [
+    "onCommand:codeServerTest.proxyUri"
+  ],
+  "engines": {
+    "vscode": "^1.56.0"
+  },
+  "main": "./extension.js",
+  "contributes": {
+    "commands": [
+      {
+        "command": "codeServerTest.proxyUri",
+        "title": "Get proxy URI",
+        "category": "code-server"
+      }
+    ]
+  },
+  "devDependencies": {
+    "@types/vscode": "^1.56.0",
+    "typescript": "^4.0.5"
+  },
+  "scripts": {
+    "build": "tsc extension.ts"
+  }
+}