REPL: call display on the backend

[IanButterworth]

Fixes #57742

With this PR (The last 2 are pasted in in one operation)

% ./julia -q -t8,0

julia> struct Foo end

julia> Base.display(::Foo) = @show Threads.threadid()

julia> Foo()
Threads.threadid() = 1

julia> Foo()
Threads.threadid() = 1

julia> Foo()
Threads.threadid() = 1

julia> Foo()
Threads.threadid() = 1

[IanButterworth]

Though I think handling errors in display might need a tweak.

julia> Base.display(::Foo) = error()

julia> Foo()

julia> Foo()

julia>

[vtjnash]

Branching on whether the parser returned an Expr or a Function seems somewhat unwise, but otherwise the PR seems a good direction