Fix the build ES6 code having the wrong file extension and use it in package.json exports

(cherry picked from commit 23d6182)

Author: diesieben07

package.json

@@ -10,7 +10,7 @@
   "repository": "https://github.com/moment/luxon",
   "exports": {
     ".": {
-      "import": "./src/luxon.js",
+      "import": "./build/es6/luxon.mjs",
       "require": "./build/node/luxon.js"
     },
     "./package.json": "./package.json"
@@ -78,8 +78,8 @@
     "build/global/luxon.js.map",
     "build/global/luxon.min.js",
     "build/global/luxon.min.js.map",
-    "build/es6/luxon.js",
-    "build/es6/luxon.js.map",
+    "build/es6/luxon.mjs",
+    "build/es6/luxon.mjs.map",
     "src"
   ],
   "license": "MIT",

tasks/build.js

@@ -86,34 +86,37 @@ async function buildLibrary(dest, opts) {
   console.log("Building", dest);
   const promises = [babelAndRollup(dest, opts)];
 
+  const filename = opts.filename || "luxon.js";
+  const minFilename = filename.replace(/\.(m?js)$/, ".min.$1");
+
   if (opts.minify && TRUST_MINIFY) {
     promises.push(
       babelAndRollup(dest, {
         ...opts,
         minify: true,
-        filename: "luxon.min.js",
+        filename: minFilename,
       })
     );
   }
 
   await Promise.all(promises);
 
   if (opts.minify && !TRUST_MINIFY) {
-    const code = fs.readFileSync(`build/${dest}/luxon.js`, "utf8"),
+    const code = fs.readFileSync(`build/${dest}/${filename}`, "utf8"),
       ugly = UglifyJS.minify(code, {
         toplevel: !opts.global,
         output: {
           comments: false,
         },
         sourceMap: {
-          filename: `build/${dest}/luxon.js`,
+          filename: `build/${dest}/${filename}`,
         },
       });
     if (ugly.error) {
       console.error("Error uglifying", ugly.error);
     } else {
-      fs.writeFileSync(`build/${dest}/luxon.min.js`, ugly.code);
-      fs.writeFileSync(`build/${dest}/luxon.min.js.map`, ugly.map);
+      fs.writeFileSync(`build/${dest}/${minFilename}`, ugly.code);
+      fs.writeFileSync(`build/${dest}/${minFilename}.map`, ugly.map);
     }
   }
   console.log("Built", dest);
@@ -153,6 +156,7 @@ async function es6() {
     format: "es",
     minify: true,
     compile: false,
+    filename: "luxon.mjs",
   });
 }