fix: add TypeScript type and documentation for markdown "mode" config (#3883)

* fix: add TypeScript type and documentation for markdown "mode" config

* chore: add changeset

.changeset/beige-lizards-scream.md

@@ -0,0 +1,5 @@
+---
+'astro': patch
+---
+
+Added "mode" to Astro config file TypeScript definitions

packages/astro/src/@types/astro.ts

@@ -513,6 +513,27 @@ export interface AstroUserConfig {
 		 */
 		drafts?: boolean;
 
+		/**
+		 * @docs
+		 * @name markdown.mode
+		 * @type {'md' | 'mdx'}
+		 * @default `mdx`
+		 * @description
+		 * Control if markdown processing is done using MDX or not.
+		 *
+		 * MDX processing enables you to use JSX inside your Markdown files. However, there may be instances where you don't want this behavior, and would rather use a "vanilla" markdown processor. This field allows you to control that behavior.
+		 *
+		 * ```js
+		 * {
+		 *   markdown: {
+		 *     // Example: Use non-MDX processor for Markdown files
+		 *     mode: 'md',
+		 *   }
+		 * }
+		 * ```
+		 */
+		mode?: 'md' | 'mdx';
+
 		/**
 		 * @docs
 		 * @name markdown.shikiConfig