fix(build): fix ESM dist file extension to .mjs

Lack of 'type' field in package.json, all .js files are treated as CJS.
To make ESM dist file treated as ESM, update dist file extension as .mjs.

Ref https://nodejs.org/api/packages.html#type

Fix #3123

config/rollup/esm.js

@@ -24,7 +24,8 @@ const plugins = [
     }),
     typescript(),
     replace({
-        "__VERSION__": version
+        "__VERSION__": version,
+        preventAssignment: true
     })
 ];
 const external = id => /^d3-/.test(id);
@@ -37,7 +38,7 @@ const bbPlugins = fs.readdirSync(path.resolve(__dirname, "../../src/Plugin/"), {
 .map(({name}) => ({
     input: `src/Plugin/${name}/index.ts`,
     output: {
-        file: `dist/plugin/billboardjs-plugin-${name}.esm.js`,
+        file: `dist/plugin/billboardjs-plugin-${name}.esm.mjs`,
         format: "es",
         banner: getBanner(true)
     },
@@ -49,7 +50,7 @@ export default [
     {
         input: "src/index.esm.ts",
         output: {
-            file: "dist/billboard.esm.js",
+            file: "dist/billboard.esm.mjs",
             format: "es",
             banner: getBanner()
         },

package.json

@@ -4,7 +4,7 @@
 	"description": "Re-usable easy interface JavaScript chart library, based on D3 v4+",
 	"homepage": "https://naver.github.io/billboard.js/",
 	"main": "dist/billboard.js",
-	"module": "dist/billboard.esm.js",
+	"module": "dist/billboard.esm.mjs",
 	"types": "types/index.d.ts",
 	"scripts": {
 		"start": "webpack-dev-server --open",