make _ by itself emit a deprecation warning

[StefanKarpinski]

Handle missing case in #20328. This change makes the way deprecation warnings get printed a bit inconsistent:

julia> _ = 1
1

julia> _
WARNING: deprecated syntax "_ as an rvalue".
Stacktrace:
 [1] depwarn(::String, ::Symbol) at ./deprecated.jl:62
 [2] eval(::Module, ::Any) at ./boot.jl:236
 [3] eval_user_input(::Any, ::Base.REPL.REPLBackend) at ./REPL.jl:66
 [4] macro expansion at ./REPL.jl:97 [inlined]
 [5] (::Base.REPL.##1#2{Base.REPL.REPLBackend})() at ./event.jl:73
while loading no file, in expression starting on line 0
1

julia> _
1

julia> _,

WARNING: deprecated syntax "_ as an rvalue".
(1,)

julia> _,

WARNING: deprecated syntax "_ as an rvalue".
(1,)

This path calls depwarn which prints a proper backtrace and only prints the first time. The other path just prints the warning with no backtrace or color and prints every time.

[JeffBezanson]

I don't think this is the correct implementation. We don't want to deprecate eval(:_), just the syntax _, in my opinion at least.

[StefanKarpinski]

Ok, I'll let you take a crack at that then. I can close this PR.