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/extension.ts

@@ -0,0 +1,13 @@
+import * as vscode from "vscode"
+
+export function activate(context: vscode.ExtensionContext) {
+  context.subscriptions.push(
+    vscode.commands.registerCommand("codeServerTest.proxyUri", () => {
+      if (process.env.VSCODE_PROXY_URI) {
+        vscode.window.showInformationMessage(process.env.VSCODE_PROXY_URI)
+      } else {
+        vscode.window.showErrorMessage("No proxy URI was set")
+      }
+    }),
+  )
+}