fix: add back marko v3 legacy helpers

.changeset/sour-carrots-ring.md

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+---
+"marko": patch
+---
+
+Add back the "legacy helpers" from Marko 3 to simplify the process of upgrading some older applications.

packages/marko/helpers/README.md

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+# Marko Helpers
+
+This directory contains helpers that were deprecated in Marko v3. Marko v4/5 no longer adds these helpers to every compiled template, but you can still import them if needed.

packages/marko/helpers/empty.js

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+var notEmpty = require("./notEmpty");
+module.exports = function empty(o) {
+  return !notEmpty(o);
+};

packages/marko/helpers/notEmpty.js

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+module.exports = function (o) {
+  if (o == null) {
+    return false;
+  } else if (Array.isArray(o)) {
+    return !!o.length;
+  } else if (o === "") {
+    return false;
+  }
+
+  return true;
+};