vim-patch:8.2.5034: there is no way to get the byte index from a virtual column

Problem:    There is no way to get the byte index from a virtual column.
Solution:   Add virtcol2col(). (Yegappan Lakshmanan, closes vim/vim#10477,
            closes vim/vim#10098)
5a6ec10cc8

Cherry-pick tv_check_for_number_arg() from Vim.
Cherry-pick pathshorten() doc change.

src/nvim/eval.lua

@@ -425,6 +425,7 @@ return {
     uniq={args={1, 3}, base=1},
     values={args=1, base=1},
     virtcol={args=1, base=1},
+    virtcol2col={args=3, base=1},
     visualmode={args={0, 1}},
     wait={args={2,3}},
     wildmenumode={},