fix: Add nixpacks configuration for Astro deployment

dealplustech-astro/node_modules/retext-stringify/lib/index.d.ts

@@ -0,0 +1,8 @@
+/**
+ * Add support for serializing natural language.
+ *
+ * @returns {undefined}
+ *   Nothing.
+ */
+export default function retextStringify(): undefined;
+export type Root = import('nlcst').Root;

dealplustech-astro/node_modules/retext-stringify/lib/index.js

@@ -0,0 +1,27 @@
+/**
+ * @typedef {import('nlcst').Root} Root
+ */
+
+import {toString} from 'nlcst-to-string'
+
+/**
+ * Add support for serializing natural language.
+ *
+ * @returns {undefined}
+ *   Nothing.
+ */
+export default function retextStringify() {
+  // eslint-disable-next-line unicorn/no-this-assignment
+  const self =
+    /** @type {import('unified').Processor<undefined, undefined, undefined, Root, string>} */ (
+      // @ts-expect-error -- TS in JSDoc doesn’t understand `this`.
+      this
+    )
+
+  self.compiler = compiler
+}
+
+/** @type {import('unified').Compiler<Root, string>} */
+function compiler(tree) {
+  return toString(tree)
+}