Fixing default devtools export

Hi! 👋

Firstly, thanks for your work on this project! 🙂

Today I used [patch-package](https://github.com/ds300/patch-package) to patch `unistore@3.5.2` for the project I'm working on.

Here is the diff that solved my problem:

```diff
diff --git a/node_modules/unistore/devtools.js b/node_modules/unistore/devtools.js
index 6c6b2f1..33143de 100644
--- a/node_modules/unistore/devtools.js
+++ b/node_modules/unistore/devtools.js
@@ -1,6 +1,6 @@
-module.exports = function unistoreDevTools(store) {
- var extension = window.__REDUX_DEVTOOLS_EXTENSION__ || window.top.__REDUX_DEVTOOLS_EXTENSION__;
- var ignoreState = false;
+function unistoreDevTools(store) {
+ const extension = window.__REDUX_DEVTOOLS_EXTENSION__ || window.top.__REDUX_DEVTOOLS_EXTENSION__;
+ const ignoreState = false;

if (!extension) {
console.warn('Please install/enable Redux devtools extension');
@@ -19,7 +19,7 @@ module.exports = function unistoreDevTools(store) {
});
store.devtools.init(store.getState());
store.subscribe(function (state, action) {
- var actionName = action && action.name || 'setState';
+ const actionName = action && action.name || 'setState';

if (!ignoreState) {
store.devtools.send(actionName, state);
@@ -31,3 +31,5 @@ module.exports = function unistoreDevTools(store) {

return store;
}
+
+export default unistoreDevTools;
```

This issue body was [partially generated by patch-package](https://github.com/ds300/patch-package/issues/296).