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-unit.sh

@@ -6,6 +6,7 @@ main() {
 
   source ./ci/lib.sh
 
+  echo "Building test plugin"
   pushd test/unit/node/test-plugin
   make -s out/index.js
   popd