feat: make sure extension hasn't been added before injecting

src/snow/index.ts

@@ -1,2 +1,14 @@
 import { Canvas } from './main/canvas'
-new Canvas()
+import { LET_IT_SNOW_CLASS } from './main/canvas/selectors'
+
+const hasCanvas = () => {
+  const canvas = document.querySelector(`.${LET_IT_SNOW_CLASS}`)
+  return !!canvas
+}
+
+const snow = () => {
+  if (hasCanvas()) return
+  new Canvas()
+}
+
+snow()