vim-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.
f273245f64

test/functional/legacy/prompt_buffer_spec.lua

@@ -0,0 +1,84 @@
+local helpers = require('test.functional.helpers')(after_each)
+local Screen = require('test.functional.ui.screen')
+local feed= helpers.feed
+local source = helpers.source
+local clear = helpers.clear
+local feed_command = helpers.feed_command
+
+describe('prompt buffer', function()
+  local screen
+
+  before_each(function()
+    clear()
+    screen = Screen.new(25, 10)
+    screen:attach()
+    source([[
+      func TextEntered(text)
+        if a:text == "exit"
+          stopinsert
+          close
+        else
+          call append(line("$") - 1, 'Command: "' . a:text . '"')
+          set nomodfied
+          call timer_start(20, {id -> TimerFunc(a:text)})
+        endif
+      endfunc
+
+      func TimerFunc(text)
+        call append(line("$") - 1, 'Result: "' . a:text .'"')
+      endfunc
+    ]])
+  end)
+
+  after_each(function()
+    screen:detach()
+  end)
+
+  it('works', function()
+    feed_command("set noshowmode | set laststatus=0")
+    feed_command("call setline(1, 'other buffer')")
+    feed_command("new")
+    feed_command("set buftype=prompt")
+    feed_command("call prompt_setcallback(bufnr(''), function('TextEntered'))")
+    screen:expect([[
+      ^                         |
+      ~                        |
+      ~                        |
+      ~                        |
+      [Scratch]                |
+      other buffer             |
+      ~                        |
+      ~                        |
+      ~                        |
+                               |
+    ]])
+    feed_command("startinsert")
+    feed("hello\n")
+    screen:expect([[
+      % hello                  |
+      Command: "hello"         |
+      Result: "hello"          |
+      % ^                       |
+      [Scratch]                |
+      other buffer             |
+      ~                        |
+      ~                        |
+      ~                        |
+                               |
+    ]])
+    feed("exit\n")
+    screen:expect([[
+      ^other buffer             |
+      ~                        |
+      ~                        |
+      ~                        |
+      ~                        |
+      ~                        |
+      ~                        |
+      ~                        |
+      ~                        |
+                               |
+    ]])
+  end)
+
+end)