Add esm build

Author: RoXuS

.gitignore

@@ -6,4 +6,5 @@ coverage
 test.js
 bench.js
 mod.d.ts
-mod.js
+mod.js
+esm

package.json

@@ -4,6 +4,7 @@
   "description": "Fastest Levenshtein distance implementation in JS.",
   "main": "mod.js",
   "types": "mod.d.ts",
+  "module": "./esm/mod.esm.js",
   "repository": {
     "type": "git",
     "url": "git+https://github.com/ka-weihe/fastest-levenshtein.git"

tsconfig.esm.json

@@ -0,0 +1,14 @@
+{
+  "compilerOptions": {
+    "target": "ES2015",
+    "module": "esnext",
+    "declaration": true,
+    "declarationMap": true,
+    "strict": true,
+    "esModuleInterop": true,
+    "skipLibCheck": true,
+    "forceConsistentCasingInFileNames": true,
+    "outDir": "esm"
+  },
+  "include": ["./mod.ts"]
+}