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/edit.pro b/src/proto/edit.pro
index 1f9e5b7..9ba7164 100644
--- a/src/proto/edit.pro
+++ b/src/proto/edit.pro
@@ -1,6 +1,8 @@
 /* edit.c */
 int edit(int cmdchar, int startln, long count);
 void edit_putchar(int c, int highlight);
+char_u *prompt_text(void);
+int prompt_curpos_editable(void);
 void edit_unputchar(void);
 void display_dollar(colnr_T col);
 void change_indent(int type, int amount, int round, int replaced, int call_changed_bytes);