use REPL ast_transforms in Julia 1.5+

[stevengj]

In Julia 1.5 or later (JuliaLang/julia#34626), you can just do

pushfirst!(Base.active_repl_backend.ast_transforms, my_transformation)

to tell the REPL to perform arbitrary AST transformations. (pushfirst! will do ast = my_transformation(ast) on each parsed ast before the default REPL.softscope transformation in Julia 1.5.)

This should be simpler than your current steal_repl_backend machinery, and more importantly it is composable with other code that wants to hook into the REPL.

[timholy]

Thanks for doing this, we've needed something like this for a long time.

[stevengj]

(Are there other REPL-stealing packages that should be using this mechanism too?)