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.

src/testdir/test_prompt_buffer.vim

@@ -0,0 +1,55 @@
+" Tests for setting 'buftype' to "prompt"
+
+if !has('channel')
+  finish
+endif
+
+source shared.vim
+source screendump.vim
+
+func Test_prompt_basic()
+  " We need to use a terminal window to be able to feed keys without leaving
+  " Insert mode.
+  if !has('terminal')
+    call assert_report('no terminal')
+    return
+  endif
+  call writefile([
+	\ 'func TextEntered(text)',
+	\ '  if a:text == "exit"',
+	\ '    stopinsert',
+	\ '    close',
+	\ '  else',
+	\ '    " Add the output above the current prompt.',
+	\ '    call append(line("$") - 1, "Command: \"" . a:text . "\"")',
+	\ '    " Reset &modified to allow the buffer to be closed.',
+	\ '    set nomodified',
+	\ '    call timer_start(20, {id -> TimerFunc(a:text)})',
+	\ '  endif',
+	\ 'endfunc',
+	\ '',
+	\ 'func TimerFunc(text)',
+	\ '  " Add the output above the current prompt.',
+	\ '  call append(line("$") - 1, "Result: \"" . a:text . "\"")',
+	\ 'endfunc',
+	\ '',
+	\ 'call setline(1, "other buffer")',
+	\ 'new',
+	\ 'set buftype=prompt',
+	\ 'call prompt_setcallback(bufnr(""), function("TextEntered"))',
+	\ 'startinsert',
+	\ ], 'Xpromptscript')
+  let buf = RunVimInTerminal('-S Xpromptscript', {})
+  call WaitForAssert({-> assert_equal('%', term_getline(buf, 1))})
+
+  call term_sendkeys(buf, "hello\<CR>")
+  call WaitForAssert({-> assert_equal('% hello', term_getline(buf, 1))})
+  call WaitForAssert({-> assert_equal('Command: "hello"', term_getline(buf, 2))})
+  call WaitForAssert({-> assert_equal('Result: "hello"', term_getline(buf, 3))})
+
+  call term_sendkeys(buf, "exit\<CR>")
+  call WaitForAssert({-> assert_equal('other buffer', term_getline(buf, 1))})
+
+  call StopVimInTerminal(buf)
+  call delete('Xpromptscript')
+endfunc