Calculate prompt token count

Problem/Motivation

It would be useful to know prompt token count, as well as, this feature is needed for other features: #3439432: Add maximum prompt length configuration and exception

Steps to reproduce

Proposed resolution

Remaining tasks

1. [done] Create utility functions with basic calculation (calculate average value with basic math)
2. [done] Display message with token count when user clicks "Display output" button
3. Do parent issue #3439432: Add maximum prompt length configuration and exception
4. At some point later, replace basic calculation with accurate calculation

User interface changes

API changes

Data model changes