vim-patch:8.2.1588: cannot read back the prompt of a prompt buffer

Problem:    Cannot read back the prompt of a prompt buffer.
Solution:   Add prompt_getprompt(). (Ben Jackson, closes vim/vim#6851)
077cc7aa0e

Updated prompt_getprompt() doc to cb80aa2d53
and removed mention of method syntax usage (not supported by Nvim).

src/nvim/eval.lua

@@ -251,6 +251,7 @@ return {
     pow={args=2},
     prevnonblank={args=1},
     printf={args=varargs(1)},
+    prompt_getprompt={args=1},
     prompt_setcallback={args={2, 2}},
     prompt_setinterrupt={args={2, 2}},
     prompt_setprompt={args={2, 2}},