Add TypeScript definition (#4)

index.d.ts

@@ -0,0 +1,29 @@
+/**
+Split a string on the first occurrence of a given separator.
+
+@param string - The string to split.
+@param separator - The separator to split on.
+
+@example
+```
+import splitOnFirst = require('split-on-first');
+
+splitOnFirst('a-b-c', '-');
+//=> ['a', 'b-c']
+
+splitOnFirst('key:value:value2', ':');
+//=> ['key', 'value:value2']
+
+splitOnFirst('a---b---c', '---');
+//=> ['a', 'b---c']
+
+splitOnFirst('a-b-c', '+');
+//=> ['a-b-c']
+```
+*/
+declare function splitOnFirst(
+	string: string,
+	separator: string
+): [string, string?];
+
+export = splitOnFirst;

index.test-d.ts

@@ -0,0 +1,4 @@
+import {expectType} from 'tsd';
+import splitOnFirst = require('.');
+
+expectType<[string, string?]>(splitOnFirst('a-b-c', '-'));

package.json

@@ -13,10 +13,11 @@
 		"node": ">=6"
 	},
 	"scripts": {
-		"test": "xo && ava"
+		"test": "xo && ava && tsd"
 	},
 	"files": [
-		"index.js"
+		"index.js",
+		"index.d.ts"
 	],
 	"keywords": [
 		"split",
@@ -28,7 +29,8 @@
 		"text"
 	],
 	"devDependencies": {
-		"ava": "^1.2.0",
+		"ava": "^1.4.1",
+		"tsd": "^0.7.2",
 		"xo": "^0.24.0"
 	}
 }