ForwardDiff of complex `erfcx` fails

Here is an observation

```julia
using SpecialFunctions
using Zygote
using ForwardDiff

f(x) = real(SpecialFunctions.erfcx((x + 1.0im)^2))

# reference:
Zygote.gradient(f, 1.0)[1] # 0.3169262912276313

# call
ForwardDiff.derivative(f, 1.0) # fails with
```
throws the error,
```julia
ERROR: MethodError: no method matching _erfcx(::Complex{ForwardDiff.Dual{ForwardDiff.Tag{var"#13#14", Float64}, Float64, 1}})
The function `_erfcx` exists, but no method is defined for this combination of argument types.
```

## Solution

It can be fixed by explaining manually how deal with complex numbers,

```julia
const ComplexDual{T, V, N} = Complex{ForwardDiff.Dual{T, V, N}}

function SpecialFunctions.erfcx(z::ComplexDual{T, V, N}) where {T, V, N}
real_part = real(z)
imag_part = imag(z)
# Get primal values
z_val = Complex(ForwardDiff.value(real_part), ForwardDiff.value(imag_part))

# Compute function value
w_val = erfcx(z_val)

# Compute derivative
∂w = 2 * (z_val * w_val - 1 / sqrt(π))

# Get partial derivatives
dr = ForwardDiff.partials(real_part)
di = ForwardDiff.partials(imag_part)

# Construct dual result
real_dual = ForwardDiff.Dual{T}(real(w_val), real(∂w) * dr - imag(∂w) * di)
imag_dual = ForwardDiff.Dual{T}(imag(w_val), imag(∂w) * dr + real(∂w) * di)

return Complex(real_dual, imag_dual)
end
```