Change polling behavior

[mattt]

Related to #106

This PR makes the following two changes to the wait method:

• Removes the max_attempts option. To stop polling for a prediction to complete, you can either return false in a callback passed to the stop parameter of the wait method, or call predictions.cancel. Or if polling via the run method, you can pass an AbortSignal to the signal property.
• Updates the polling interval for a prediction from 250ms to 500ms. This is a more reasonable default for most use cases, and it brings this project in line with the Python client's behavior.