refactor: move some structs out of buffer_defs.h (#24878)

src/nvim/arglist_defs.h

@@ -0,0 +1,22 @@
+#ifndef NVIM_ARGLIST_DEFS_H
+#define NVIM_ARGLIST_DEFS_H
+
+#include "nvim/garray.h"
+
+/// Argument list: Array of file names.
+/// Used for the global argument list and the argument lists local to a window.
+typedef struct arglist {
+  garray_T al_ga;   ///< growarray with the array of file names
+  int al_refcount;  ///< number of windows using this arglist
+  int id;           ///< id of this arglist
+} alist_T;
+
+/// For each argument remember the file name as it was given, and the buffer
+/// number that contains the expanded file name (required for when ":cd" is
+/// used).
+typedef struct argentry {
+  char *ae_fname;  ///< file name as specified
+  int ae_fnum;     ///< buffer number with expanded file name
+} aentry_T;
+
+#endif  // NVIM_ARGLIST_DEFS_H