Set and seek marks for entries in REPL history

[caleb-allen]

For example, setting a mark mA would allow a user to return to a given history entry using 'A.

One open question: Should marks be allowed to be set when browsing the history? Or should they only be able to be set when actively writing text in the repl (in other words, not browsing through REPL history)

[caleb-allen]

A possible implementation of this would be a mark dictionary (persisted with Preferences.jl) where the dictionary's key is the mark's name (the keyboard key) and the value is the date/timestamp of the history entry, which is already recorded in the history file