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>
