patch 8.2.1054: not so easy to pass a lua function to Vim

Problem:    Not so easy to pass a lua function to Vim.
Solution:   Convert a Lua function and closure to a Vim funcref. (Prabir
            Shrestha, closes #6246)

src/structs.h

@@ -1529,6 +1529,9 @@ struct blobvar_S
     char	bv_lock;	// zero, VAR_LOCKED, VAR_FIXED
 };
 
+typedef int (*cfunc_T)(int argcount, typval_T *argvars, typval_T *rettv, void *state);
+typedef void (*cfunc_free_T)(void *state);
+
 #if defined(FEAT_EVAL) || defined(PROTO)
 typedef struct funccall_S funccall_T;
 
@@ -1562,6 +1565,11 @@ typedef struct
     char_u	*uf_va_name;	// name from "...name" or NULL
     type_T	*uf_va_type;	// type from "...name: type" or NULL
     type_T	*uf_func_type;	// type of the function, &t_func_any if unknown
+# if defined(FEAT_LUA)
+    cfunc_T     uf_cb;		// callback function for cfunc
+    cfunc_free_T uf_cb_free;    // callback function to free cfunc
+    void        *uf_cb_state;   // state of uf_cb
+# endif
 
     garray_T	uf_lines;	// function lines
 # ifdef FEAT_PROFILE
@@ -1607,6 +1615,7 @@ typedef struct
 #define FC_EXPORT   0x100	// "export def Func()"
 #define FC_NOARGS   0x200	// no a: variables in lambda
 #define FC_VIM9	    0x400	// defined in vim9 script file
+#define FC_CFUNC    0x800	// defined as Lua C func
 
 #define MAX_FUNC_ARGS	20	// maximum number of function arguments
 #define VAR_SHORT_LEN	20	// short variable name length