feat: add basic typescript module declaration

Thanks to @losfroger and @jim-toth in the related issue. This implementation
was a merge of their recommended fixes and the official Vue 3 Options API
Type Augmentation documentation.

I've tested the module definitions against a real-world Vue 3 codebase in both
Composition and Options API mode.

Closes #15.

dist/index.d.ts

@@ -0,0 +1,13 @@
+import Shepherd from 'shepherd.js';
+import { ObjectPlugin } from 'vue';
+
+declare module 'vue-shepherd' {
+    export function useShepherd(...args: Array<Shepherd.Tour.TourOptions>): Shepherd.Tour;
+    export default ObjectPlugin;
+}
+
+declare module 'vue' {
+    interface ComponentCustomProperties {
+        $shepherd: (...args: Array<Shepherd.Tour.TourOptions>) => Shepherd.Tour;
+    }
+}

package.json

@@ -1,6 +1,6 @@
 {
   "name": "vue-shepherd",
-  "version": "3.0.0",
+  "version": "3.1.0",
   "description": "A Vue wrapper for the site tour library Shepherd.",
   "author": {
     "name": "Robert Wagner",
@@ -29,10 +29,13 @@
     "dist/vue-shepherd.esm.js.map",
     "dist/vue-shepherd.min.js",
     "dist/vue-shepherd.ssr.js",
+    "dist/index.d.ts",
     "package.json",
     "README.md",
-    "src"
+    "src",
+    "tsconfig.json"
   ],
+  "types": "./dist/index.d.ts",
   "dependencies": {
     "shepherd.js": "^11.2.0"
   },