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fix: re-parsable printing of vararg anon func defs (#54019)

Currently, anonymous function definitions with one slurping argument are
not printed in a re-parsable way:
```
julia> Meta.parse("(a...) -> a")
:(a...->begin
          #= none:1 =#
          a
      end)

julia> Meta.parse(repr(Meta.parse("(a...) -> a")))
ERROR: ParseError:
# Error @ none:1:7
#     ┌──────
:(a...->begin
          #= none:1 =#
          a
#─────────┘ ── Expected `)`
Stacktrace:
 [1] #parse#3
   @ ./meta.jl:244 [inlined]
 [2] parse
   @ ./meta.jl:236 [inlined]
 [3] parse(str::String; filename::String, raise::Bool, depwarn::Bool)
   @ Base.Meta ./meta.jl:278
 [4] parse(str::String)
   @ Base.Meta ./meta.jl:276
 [5] top-level scope
   @ REPL[2]:1
   ```

This PR wraps the argument list in a `(:tuple, ...)`, much like what we are already doing for normal functions:
```
julia> Meta.parse("function (a...) end")
:(function (a...,)
      #= none:1 =#
      #= none:1 =#
  end)
```

Technically neither of those are entirely correct (since they're not idempotent). A fix for that is more involved, but I can take a look at that as well if necessary.

---------

Co-authored-by: Andy Dienes <51664769+adienes@users.noreply.github.com>

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