vim-patch:8.2.2957: using getchar() in Vim9 script is problematic

Problem:    Using getchar() in Vim9 script is problematic.
Solution:   Add getcharstr(). (closes vim/vim#8343)
3a7503c34c

Cherry-pick Test_getchar() changes from patch v8.1.2304
to sync with upstream.

Port f_getcharstr() to src/nvim/eval/funcs.c, not src/nvim/getchar.c.
Patch v8.1.2042 is not ported yet.

src/nvim/eval.lua

@@ -136,6 +136,7 @@ return {
     getchar={args={0, 1}},
     getcharmod={},
     getcharsearch={},
+    getcharstr={args={0, 1}},
     getcmdline={},
     getcmdpos={},
     getcmdtype={},