Apply suggestions from code review

Co-authored-by: David Graham <dgraham@github.com>

src/index.ts

@@ -41,7 +41,7 @@ function getButtons(toolbar: Element): HTMLElement[] {
   return els
 }
 
-function keydown(fn: (arg0: KeyboardEvent) => void): (arg0: KeyboardEvent) => void {
+function keydown(fn: (event: KeyboardEvent) => void): (event: KeyboardEvent) => void {
   return function (event: KeyboardEvent) {
     if (event.key === ' ' || event.key === 'Enter') {
       event.preventDefault()

tsconfig.json

@@ -1,6 +1,6 @@
 {
   "compilerOptions": {
-    "module": "esnext",
+    "module": "es2020",
     "target": "es2017",
     "strict": true,
     "declaration": true,