Move spread directive to framework library

framework/src/index.ts

@@ -1 +1,2 @@
 export * from "./lib/decorators/on";
+export { default as spread } from "./lib/directives/spread";

framework/src/lib/directives/spread.ts

@@ -0,0 +1,44 @@
+import { ElementPart, nothing } from "lit";
+import { directive, Directive } from "lit/directive.js";
+
+interface Stringable {
+	toString(): string;
+}
+
+/**
+ * A simple implementation of React's spread operator (`...`) for Lit. Assigns
+ * each `{ [key]: value }` entry in the provided object as an HTML attribute to
+ * the element it's invoked on.
+ *
+ * React:
+ * @example
+ * ```jsx
+ * export default (props) => <div {...props} />
+ * ```
+ *
+ * Lit:
+ * @example
+ * ```typescript
+ * export class FooElement extends LitElement {
+ *   props: Record<string, string> = {};
+ *   override render = () => html`<div ${spread(props)}></div>`
+ * }
+ * ```
+ *
+ * @note
+ * The object entries are assigned as HTML _attributes_, not _properties_, so
+ * each enumerable value in the provided object must have a meaningnful
+ * `toString()` implementation.
+ */
+export default directive(class Spread extends Directive {
+	override render(_: Record<string, Stringable>) {
+		return nothing;
+	}
+
+	override update(part: ElementPart, [attrs]: [Record<string, Stringable>]) {
+		for (let [key, value] of Object.entries(attrs)) {
+			part.element.setAttribute(key, value.toString());
+		}
+		return part;
+	}
+});