Implement Command Mode using REPL AST Transforms

[caleb-allen]

VimBindings.jl may benefit from the new REPL AST transforms feature in the Julia REPL, described at about 17:00 in this video: https://youtu.be/qM9NtiYlXck

My first idea is that a new REPL mode could be added, similar to Shell mode or Help mode, but this one is instead only accessible by striking the : key while in the VimBindings normal mode. In this case, the text currently in the REPL should be saved, and restored after the command is run, depending on the action of the command.

REPL AST transforms could then be used to implement a subset of vim commands. For example

• search with / or backwards search with ?

[caleb-allen]

Issue related to documenting REPL AST transforms here JuliaLang/julia#37047