feat: Add a configuration for an instance that can be set at creation time

And use it to control whether the result of a complex operation where the
   imaginary part comes out to 0 will be coerced back to a number.

complex/add.js

@@ -2,6 +2,7 @@ import { anyComplex } from './complex.js'
 
 export default function create(pmath) {
     const complex = pmath('complex')
+    const resolve = pmath.config.resolveComplex
     return pmath('add', [anyComplex, // naive, but this is just a P-o-C
        (...addends) => {
            let sum = complex(addends[0])
@@ -10,6 +11,7 @@ export default function create(pmath) {
                sum.re += addend.re
                sum.im += addend.im
            }
+           if (resolve && Math.abs(sum.im/sum.re) < 1e-10) return sum.re
            return sum
        }])
 }