Add TypeScript definition (#12)

Co-authored-by: Sindre Sorhus <sindresorhus@gmail.com>

index.d.ts

@@ -0,0 +1,19 @@
+/**
+ * use typescript
+ * Fix the $PATH on macOS when run from a GUI app
+ * @example
+ * ```
+ * import fixPath from 'fix-path';
+ * 
+ * console.log(process.env.PATH);
+ * //=> '/usr/bin'
+ * 
+ * fixPath();
+ * 
+ * console.log(process.env.PATH);
+ * //=> '/usr/local/bin:/usr/bin'
+ * 
+ * ```
+ */
+
+export default function fixPath(): void; 

index.test-d.ts

@@ -0,0 +1,4 @@
+import {expectType} from 'tsd';
+import fixPath from '.';
+
+expectType<void>(fixPath());

package.json

@@ -13,10 +13,11 @@
 		"node": ">=8"
 	},
 	"scripts": {
-		"test": "xo && ava"
+		"test": "xo && ava && tsd"
 	},
 	"files": [
-		"index.js"
+		"index.js",
+		"index.d.ts"
 	],
 	"keywords": [
 		"fix",
@@ -36,6 +37,8 @@
 	},
 	"devDependencies": {
 		"ava": "^1.4.1",
+		"tsd": "^0.11.0",
 		"xo": "^0.24.0"
-	}
+	},
+	"types": "index.d.ts"
 }