Bundle brainstorm server dependencies instead of requiring npm install

Vendor node_modules into the repo so the brainstorm server works
immediately on fresh plugin installs without needing npm at runtime.

skills/brainstorming/scripts/node_modules/math-intrinsics/sign.js

@@ -0,0 +1,11 @@
+'use strict';
+
+var $isNaN = require('./isNaN');
+
+/** @type {import('./sign')} */
+module.exports = function sign(number) {
+	if ($isNaN(number) || number === 0) {
+		return number;
+	}
+	return number < 0 ? -1 : +1;
+};