Add FlashMessage TS definition (#405)

Author: charlesfries

ember-cli-flash/declarations/components/flash-message.d.ts

@@ -0,0 +1,16 @@
+declare module 'ember-cli-flash/components/flash-message' {
+  import Component from '@glimmer/component';
+  import FlashObject from 'ember-cli-flash/flash/object';
+
+  export default class FlashMessage extends Component<{
+    Args: {
+      flash: FlashObject;
+      messageStyle?: 'bootstrap' | 'foundation';
+      messageStylePrefix?: string;
+    };
+    Blocks: {
+      default: [FlashMessage, FlashObject, () => void];
+    };
+    Element: HTMLDivElement;
+  }> {}
+}