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

Problem:    Using getchar() in Vim9 script is problematic.
Solution:   Add getcharstr(). (closes #8343)
diff --git a/src/proto/getchar.pro b/src/proto/getchar.pro
index cb8b750..25aef1c 100644
--- a/src/proto/getchar.pro
+++ b/src/proto/getchar.pro
@@ -46,6 +46,7 @@
 int vpeekc_any(void);
 int char_avail(void);
 void f_getchar(typval_T *argvars, typval_T *rettv);
+void f_getcharstr(typval_T *argvars, typval_T *rettv);
 void f_getcharmod(typval_T *argvars, typval_T *rettv);
 void parse_queued_messages(void);
 void vungetc(int c);