Add prompt string support for REPL

[daviwil]

The Editor Services REPL should provide prompt string, much like what you see in the ISE today:

PS C:\Users\daviwil>

This prompt gets updated based on the state of the user's session. For example, when script execution breaks into the debugger, the prompt gets a [DBG] prefix:

[DBG]: PS C:\Users\daviwil>>

The prompt implementation should also support custom user prompts my invoking the prompt function to get the user's prompt string.