Fix method show with kwarg and optional arguments

[Liozou]

@vtjnash I believe this is what you actually indended in the base/methodshow.jl edit of #48639?
Otherwise, currently there is the odd var""... that appears when showing methods (and thus in REPL completion):

julia> function foo(a=12.0; b) end;

julia> foo(#TAB, on master
foo(a; b) @ Main REPL[1]:1        foo(; var""...) @ Main REPL[1]:1

julia> foo(#TAB, this PR
foo(a; b) @ Main REPL[1]:1   foo(; ...) @ Main REPL[1]:1