Add node-esm test's index.mjs, remove .gitignore rules that blocked it in the first place

Author: mfedderly

.gitignore

@@ -1,14 +1,7 @@
 .rpt2_cache
 packages/*/dist
-main.js
-main.es.js
-main.mjs
-main.min.js
-index.mjs
 *.min.js
-*.es5.js
 .esm-cache
-package-lock.json
 env
 .nyc_output
 lib-cov

test/node-esm/index.mjs

@@ -0,0 +1,15 @@
+import * as turf from "@turf/turf";
+
+const polygon = turf.polygon([
+  [
+    [125, -15],
+    [113, -22],
+    [154, -27],
+    [144, -15],
+    [125, -15],
+  ],
+]);
+
+const area = turf.area(polygon);
+
+console.log(area);