vim-patch:8.1.1111: it is not easy to check for infinity

Problem:    It is not easy to check for infinity.
Solution:   Add isinf(). (Ozaki Kiichi, closes vim/vim#3787)
fda1bff39f

src/nvim/eval.lua

@@ -189,6 +189,7 @@ return {
     insert={args={2, 3}},
     invert={args=1},
     isdirectory={args=1},
+    isinf={args=1},
     islocked={args=1},
     isnan={args=1},
     id={args=1},