fix(overmind): make reaction an instance propertty

Author: christianalfoni

packages/node_modules/overmind/src/index.ts

@@ -826,11 +826,11 @@ export class Overmind<ThisConfig extends IConfiguration>
   getMutationTree(): IMutationTree<any> {
     return this.proxyStateTree.getMutationTree()
   }
-  reaction(
+  reaction = (
     stateCallback: (state: ThisConfig['state']) => any,
     updateCallback: (state: ThisConfig['state']) => void,
     deep: boolean = false
-  ) {
+  ) => {
     if (deep) {
       const value = stateCallback(this.state)