feat(--headless): add on_print callback to stdioopen

This commit adds an on_print callback to stdioopen's dictionary
argument which lets the caller specify a function called each time
neovim will try to output something to stdout (e.g. on "echo" or
"echoerr" in --headless mode).

src/nvim/channel.h

@@ -96,6 +96,8 @@ struct Channel {
 
 EXTERN PMap(uint64_t) channels INIT(= MAP_INIT);
 
+EXTERN Callback on_print INIT(= CALLBACK_INIT);
+
 #ifdef INCLUDE_GENERATED_DECLARATIONS
 # include "channel.h.generated.h"
 #endif