Add TypeScript typings

Issue: mathiasbynens/windows-1252#6

package.json

@@ -30,8 +30,10 @@
 	"bugs": "https://github.com/mathiasbynens/koi8-r/issues",
 	"files": [
 		"LICENSE-MIT.txt",
-		"koi8-r.mjs"
+		"koi8-r.mjs",
+		"koi8-r.d.ts"
 	],
+	"types": "koi8-r.d.ts",
 	"scripts": {
 		"download": "curl https://encoding.spec.whatwg.org/index-koi8-r.txt > data/index.txt",
 		"build": "node scripts/transform-data.js",

src/koi8-r.d.ts

@@ -0,0 +1,18 @@
+declare module 'koi8-r' {
+  type EncodeOptions = {
+    mode: 'fatal' | 'replacement';
+  };
+  type DecodeOptions = {
+    mode: 'fatal' | 'replacement';
+  };
+
+  export declare function encode(
+    text: string,
+    options?: EncodeOptions
+  ): Uint16Array;
+  export declare function decode(
+    buffer: Uint16Array | Uint8Array | Buffer | string,
+    options?: DecodeOptions
+  ): string;
+  export type labels = string[];
+}