Calls the browserless.io service (local instance can be spawned with make browserless) and runs verification/verify_plausible_installed.js via the function API.

The successful execution assumes the following JSON payload:

• data.plausibleInstalled - boolean indicating whether the plausible() window function was found
• data.callbackStatus - integer. 202 indicates that the server acknowledged the test event.

The test event ingestion is discarded based on user-agent, see: Plausible.Verification.user_agent/0