refactor: move deps declarations into src (#5946)

packages/docusaurus-mdx-loader/src/deps.d.ts

@@ -0,0 +1,29 @@
+/**
+ * Copyright (c) Facebook, Inc. and its affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+// TODO Types provided by MDX 2.0 https://github.com/mdx-js/mdx/blob/main/packages/mdx/types/index.d.ts
+declare module '@mdx-js/mdx' {
+  import type {Processor} from 'unified';
+  import type {RemarkOrRehypePlugin} from '@docusaurus/mdx-loader';
+
+  namespace mdx {
+    interface Options {
+      filepath?: string;
+      skipExport?: boolean;
+      wrapExport?: string;
+      remarkPlugins?: RemarkOrRehypePlugin[];
+      rehypePlugins?: RemarkOrRehypePlugin[];
+    }
+
+    function sync(content: string, options?: Options): string;
+    function createMdxAstCompiler(options?: Options): Processor;
+    function createCompiler(options?: Options): Processor;
+  }
+  function mdx(content: string, options?: mdx.Options): Promise<string>;
+
+  export default mdx;
+}