Make the REPL prompt customizable

[sviolich]

Proposal

I propose making the REPL prompt customizable, so the user can more easily find their previous questions and answers when scrolling back over a long REPL session.

The user will be able to change the prompt characters, add newlines for whitespace, and add color using the Rich library:

Current State

The REPL prompt is just >>>, and previous questions and answers can be hard to spot:

Details

I've made a prototype already. I changed ReplHandler to use a rich.prompt.Prompt. The user can specify a new REPL prompt by adding this variable to the config file:

REPL_PROMPT="\n[white on grey] >>> [/white on grey] "