implement pre-run REPL hooks.

[jlapeyre]

Implemented user hooks that are called after the REPL is created and setup, just before starting the REPL loop.

In .juliarc.jl, you can include code like this:

if isdefined(Base, :atreplrun)
Base.atreplrun( (repl)->repl.interface.modes[1].prompt = "newprompt>" )
end

to install a hook. Call atreplrun repeatedly to push hooks, all of which are run (more or less) just before the loop is entered.

Some people think this is useful. But, considering WIP: Add hook API to REPL #13545, maybe a more general solution is needed.

Committer: John Lapeyre lapeyre@ribot
On branch gjl/replhooks
Changes to be committed:
modified: base/REPL.jl
modified: base/client.jl

[vtjnash]

run_frontend isn't generically coupled to the REPL started by Base._start. we already have _atreplinit, to handle some cases. other callbacks would need to be implemented as fields of the appropriate REPL type.