feat: Add complex numbers

With just the operations we have for numbers, and overall tests.

test/_picomath.js

@@ -12,4 +12,12 @@ describe('The default full picomath instance "math"', () => {
          (...addends) => addends.reduce((x,y) => x+y, '')])
       assert.strictEqual(math.add('Kilroy',' is here'), 'Kilroy is here')
    })
+
+   it('handles complex numbers', () => {
+      assert.deepStrictEqual(math.complex(2,3), {re: 2, im: 3})
+      assert.deepStrictEqual(
+         math.subtract(16, math.add(3, math.complex(0,4), 2)),
+         math.complex(11, -4))
+      assert.deepStrictEqual(math.negate(math.complex(3, '8')).im, -8)
+   })
 })