vim-patch:8.1.1291: not easy to change directory and restore

Problem:    Not easy to change directory and restore.
Solution:   Add the chdir() function. (Yegappan Lakshmanan, closes vim/vim#4358)
1063f3d200

Also includes some documentation changes from patch 8.1.1218.

src/nvim/eval.lua

@@ -72,6 +72,7 @@ return {
     chansend={args=2},
     char2nr={args={1, 2}, base=1},
     charidx={args={2, 3}},
+    chdir={args=1, base=1},
     cindent={args=1, base=1},
     clearmatches={args={0, 1}, base=1},
     col={args=1, base=1},