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/playwright.config.ts

@@ -25,12 +25,10 @@ const config: PlaywrightTestConfig = {
       name: "Chromium",
       use: { browserName: "chromium" },
     },
-
     {
       name: "Firefox",
       use: { browserName: "firefox" },
     },
-
     {
       name: "WebKit",
       use: { browserName: "webkit" },