fix: TypeScript file extension should be .ts (#101)

Author: rzhw

package.json

@@ -12,7 +12,7 @@
   "homepage": "https://github.com/shellscape/postcss-values-parser",
   "bugs": "https://github.com/shellscape/postcss-values-parser/issues",
   "main": "lib/index.js",
-  "types": "lib/index.d.js",
+  "types": "lib/index.d.ts",
   "engines": {
     "node": ">=6.14.4"
   },