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.

ci/dev/test-e2e.sh

@@ -13,6 +13,11 @@ main() {
 
   source ./ci/lib.sh
 
+  pushd test/e2e/extensions/test-extension
+  echo "Building test extension"
+  yarn build
+  popd
+
   local dir="$PWD"
   if [[ ! ${CODE_SERVER_TEST_ENTRY-} ]]; then
     echo "Set CODE_SERVER_TEST_ENTRY to test another build of code-server"