chore: implement Fq add (#9354)

This is more changes for the new address scheme.
I think it was just missing here. We need this to be able to add our
preaddress (h) to our ivsk, to compute our addressSecret.

yarn-project/foundation/src/fields/fields.ts

@@ -381,6 +381,10 @@ export class Fq extends BaseField {
     return new Fq((high.toBigInt() << Fq.HIGH_SHIFT) + low.toBigInt());
   }
 
+  add(rhs: Fq) {
+    return new Fq((this.toBigInt() + rhs.toBigInt()) % Fq.MODULUS);
+  }
+
   toJSON() {
     return {
       type: 'Fq',