feat: cmdline funcs (#18284)

vim-patch:8.2.4903: cannot get the current cmdline completion type and position

Problem:    Cannot get the current cmdline completion type and position.
Solution:   Add getcmdcompltype() and getcmdscreenpos(). (Shougo Matsushita,
            closes vim/vim#10344)
79d599b877

vim-patch:8.2.4910: imperfect coding

Problem:    Imperfect coding.
Solution:   Make code nicer.
9ff7d717aa

src/nvim/eval.lua

@@ -152,8 +152,10 @@ return {
     getcharpos={args=1, base=1},
     getcharsearch={},
     getcharstr={args={0, 1}},
+    getcmdcompltype={},
     getcmdline={},
     getcmdpos={},
+    getcmdscreenpos={},
     getcmdtype={},
     getcmdwintype={},
     getcompletion={args={2, 3}, base=1},