Add TypeScript definition (#16)

index.d.ts

@@ -0,0 +1,7 @@
+/**
+Check if a string is HTML.
+
+@param string - The string to check if it is HTML.
+@returns `true` if the string is HTML, `false` otherwise.
+*/
+export default function isHtml(string: string): boolean;

package.json

@@ -12,14 +12,16 @@
 	},
 	"type": "module",
 	"exports": "./index.js",
+	"types": "./index.d.ts",
 	"engines": {
 		"node": "^12.20.0 || ^14.13.1 || >=16.0.0"
 	},
 	"scripts": {
 		"test": "xo && ava"
 	},
 	"files": [
-		"index.js"
+		"index.js",
+		"index.d.ts"
 	],
 	"keywords": [
 		"html",