feat(terminal): parse current buffer contents in nvim_open_term() (#33720)

When nvim_open_term() is called with a non-empty buffer, the buffer
contents are piped into the PTY.

src/nvim/buffer.h

@@ -8,6 +8,7 @@
 #include "nvim/gettext_defs.h"  // IWYU pragma: keep
 #include "nvim/macros_defs.h"
 #include "nvim/marktree_defs.h"
+#include "nvim/strings.h"
 #include "nvim/types_defs.h"
 
 /// Values for buflist_getfile()