feat(nvim_open_term): support input callback in lua

src/nvim/channel.h

@@ -42,6 +42,10 @@ typedef struct {
   bool closed;
 } StderrState;
 
+typedef struct {
+  LuaRef cb;
+} InternalState;
+
 typedef struct {
   Callback cb;
   dict_T *self;
@@ -74,6 +78,7 @@ struct Channel {
     Stream socket;
     StdioPair stdio;
     StderrState err;
+    InternalState internal;
   } stream;
 
   bool is_rpc;