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/Makefile b/src/Makefile
index 0e4c555..ca97254 100644
--- a/src/Makefile
+++ b/src/Makefile
@@ -2252,6 +2252,7 @@
 	test_popup \
 	test_preview \
 	test_profile \
+	test_prompt_buffer \
 	test_put \
 	test_python2 \
 	test_python3 \