feat: add package exports for ES modules

This commit adds the "exports" field to package.json, enabling the
package to be imported as an ES module. The "import" field points to
the compiled JavaScript file, and the "types" field points to the
TypeScript definition file.

Author: ryoppippi

package.json

@@ -13,6 +13,12 @@
 		"css",
 		"css-modules"
 	],
+	"exports": {
+		".": {
+			"types": "./dist/index.d.ts",
+			"import": "./dist/index.mjs"
+		}
+	},
 	"files": [
 		"dist",
 		"src"