patch 8.1.0027: difficult to make a plugin that feeds a line to a job

Problem:    Difficult to make a plugin that feeds a line to a job.
Solution:   Add the nitial code for the "prompt" buftype.
diff --git a/src/proto/ops.pro b/src/proto/ops.pro
index 13e063e..01df56f 100644
--- a/src/proto/ops.pro
+++ b/src/proto/ops.pro
@@ -1,6 +1,7 @@
 /* ops.c */
 int get_op_type(int char1, int char2);
 int op_on_lines(int op);
+int op_is_change(int op);
 int get_op_char(int optype);
 int get_extra_op_char(int optype);
 void op_shift(oparg_T *oap, int curs_top, int amount);