Add import-map-overrides:init event. (#49)

* Add import-map-overrides:init event.

* Docs

docs/api.md

@@ -260,6 +260,18 @@ window.importMapOverrides.isExternalMapValid(
 
 ### Events
 
+#### Init
+
+The import-map-overrides library fires an event called `import-map-overrides:init` on the window when the library has successfully initialized. Note that this event will not fire if import-map-overrides is disabled.
+
+```sh
+window.addEventListener("import-map-overrides:init", () => {
+  console.log('init');
+})
+```
+
+#### Change
+
 The import-map-overrides library fires an event called `import-map-overrides:change` on the window whenever the
 override import map changes. The event is a [CustomEvent](https://developer.mozilla.org/en-US/docs/Web/API/CustomEvent)
 that has no [detail property](https://developer.mozilla.org/en-US/docs/Web/API/CustomEvent/detail).

src/api/js-api.js

@@ -365,11 +365,17 @@ function init() {
   }
 
   function fireChangedEvent() {
+    fireEvent("change");
+  }
+
+  function fireEvent(type) {
     // Set timeout so that event fires after the change has totally finished
     setTimeout(() => {
-      if (canFireCustomEvents) {
-        window.dispatchEvent(new CustomEvent("import-map-overrides:change"));
-      }
+      const eventType = `import-map-overrides:${type}`;
+      const event = canFireCustomEvents
+        ? new CustomEvent(eventType)
+        : document.createEvent(eventType);
+      window.dispatchEvent(event);
     });
   }
 
@@ -425,6 +431,8 @@ function init() {
     }
   }
 
+  fireEvent("init");
+
   function insertOverrideMap(map, id, referenceNode) {
     const overrideMapElement = document.createElement("script");
     overrideMapElement.type = importMapType;

test/embedded-map.html

@@ -16,6 +16,11 @@
         }
       }
     </script>
+    <script>
+      window.addEventListener("import-map-overrides:init", () => {
+        console.log("IMO init fired");
+      });
+    </script>
     <script src="/import-map-overrides.js"></script>
     <script src="https://cdn.jsdelivr.net/npm/systemjs/dist/system.js"></script>
   </head>