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.ts

@@ -0,0 +1,12 @@
+import { describe, test } from "./baseFixture"
+
+describe("Extensions", true, () => {
+  // This will only work if the test extension is loaded into code-server.
+  test("should have access to VSCODE_PROXY_URI", async ({ codeServerPage }) => {
+    const address = await codeServerPage.address()
+
+    await codeServerPage.executeCommandViaMenus("code-server: Get proxy URI")
+
+    await codeServerPage.page.waitForSelector(`text=${address}/proxy/{{port}}`)
+  })
+})