Warn about function definitions with grouping parens

Function definitions with grouping parentheses around the signature are ugly and somewhat syntactically ambiguous. We must support them for compatibility but I think they're a syntactic misfeature we should discourage with a warning (and eventually in the far future disallow).

---

A first example of ambiguity

```julia
# 0-arg function named `f`
function (f())
end

# 1-arg anonymous function
function (f)
end

# function declaration without methods
function f
end
```

Arguably, the first form could be seen as a 1-arg anonymous function (with invalid nested expression `f()` which would be caught at lowering).

Allowing grouping parentheses here isn't ever necessary but has been done several times in the package ecosystem. The only valid use case I can think of is wanting to put the `where` on a new line (https://github.com/JuliaLang/JuliaSyntax.jl/issues/116#issuecomment-1271096904) but this is not really necessary.

---

There's a further problem about the precedence of `::` and `where` sufficies to the call expression. To copy some comments from https://github.com/JuliaLang/JuliaSyntax.jl/pull/131#issuecomment-1295364086

You can't "just add grouping parens" without changing the relative precedence of `::` and `where`:

Ie, we can't go from the normal syntax

```julia
julia> function f(a::T)::T where T
a
end
f (generic function with 1 method)

julia> f(1)
1
```

To the same thing with parens without the precedence of `::` and `where` changing:

```julia
julia> function (g(a::T)::T where T)
a
end
ERROR: UndefVarError: T not defined
```

Additional parens (like in your example) are required to manually fix the precedence:

```julia
julia> function ((h(a::T)::T) where T)
a
end
h (generic function with 1 method)
```