Add first test

Why:

- Every journey starts with a first step

This change addresses the need by:

- Adding jest and a test
- Resolve ESLint warnings/errors in the tested function

src/litegraph.test.js

@@ -0,0 +1,15 @@
+const lg = require("./litegraph");
+
+test("Register a node type", () => {
+    function calc_sum(a, b) {
+        return a + b;
+    }
+    lg.LiteGraph.registerNodeType("math/sum", calc_sum);
+
+    let node = lg.LiteGraph.registered_node_types["math/sum"];
+    expect(node).toBeTruthy();
+    expect(node.type).toBe("math/sum");
+    expect(node.title).toBe("calc_sum")
+    expect(node.category).toBe("math");
+    expect(node.prototype.configure).toBe(lg.LGraphNode.prototype.configure);
+});