Problem: Using getchar() in Vim9 script is problematic.
Solution: Add getcharstr(). (closesvim/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.