Use rollup and split up files

src/Layer.js

@@ -0,0 +1,14 @@
+// @ts-check
+
+class Layer {
+	/**
+	 * @param {string[][]} pixels
+	 * @param {string} [tag]
+	 */
+	constructor(pixels, tag = "default") {
+		this.pixels = pixels;
+		this.tag = tag;
+	}
+}
+
+export default Layer;