Publish JavaScript modules (ESM)

ESM module are published unminified by convention.
Therefore they can simply be build by `tsc`.

We target ES6 for now to comply with the
browser level that flatpickr itself supports.
See flatpickr/flatpickr#2468
for more information.

package.json

@@ -4,19 +4,22 @@
   "version": "0.4.0",
   "exports": {
     "types": "./dist/types/index.d.ts",
+    "import": "./dist/esm/index.js",
     "default": "./dist/shortcut-buttons-flatpickr.js"
   },
   "main": "dist/shortcut-buttons-flatpickr.js",
+  "module": "dist/esm/index.js",
   "types": "dist/types/index.d.ts",
   "scripts": {
     "build": "run-s build:pre build:dev build:prod",
     "build:pre": "rm -rf dist && mkdir -p dist/themes",
     "build:dev": "run-s build:ts:dev build:themes:dev",
-    "build:prod": "run-s build:ts:prod build:types build:themes:prod",
+    "build:prod": "run-s build:ts:prod build:esm build:types build:themes:prod",
     "build:ts:dev": "webpack --config build/webpack.config.ts --mode development",
     "build:ts:prod": "webpack --config build/webpack.config.ts --mode production",
     "build:themes:dev": "webpack --config build/webpack.themes.ts --mode development",
     "build:themes:prod": "webpack --config build/webpack.themes.ts --mode production",
+    "build:esm": "tsc -p tsconfig.esm.json",
     "build:types": "tsc -p tsconfig.declarations.json",
     "prepublishOnly": "run-s build test",
     "start:server": "forever start -m 1 -c ts-node tests/server.ts",

tsconfig.esm.json

@@ -0,0 +1,11 @@
+{
+  "compilerOptions": {
+    "target": "es5",
+    "module": "es6",
+    "outDir": "dist/esm"
+  },
+  "extends": "./tsconfig.json",
+  "include": [
+    "./src/index.ts"
+  ]
+}