vim-patch:8.2.2233: cannot convert a byte index into a character index (#13978)

Problem:    Cannot convert a byte index into a character index.
Solution:   Add charidx(). (Yegappan Lakshmanan, closes vim/vim#7561)
17793ef23a

src/nvim/eval.lua

@@ -63,6 +63,7 @@ return {
     chanclose={args={1, 2}},
     chansend={args=2},
     char2nr={args={1, 2}},
+    charidx={args={2, 3}},
     cindent={args=1},
     clearmatches={args={0, 1}},
     col={args=1},