patch 8.2.1977: Vim9: error for using a string in a condition is confusing

Problem:    Vim9: error for using a string in a condition is confusing.
Solution:   Give a more specific error.  Also adjust the compile time type
            checking for || and &&.
diff --git a/src/proto/vim9execute.pro b/src/proto/vim9execute.pro
index 2f0ef54..882456d 100644
--- a/src/proto/vim9execute.pro
+++ b/src/proto/vim9execute.pro
@@ -4,5 +4,6 @@
 int call_def_function(ufunc_T *ufunc, int argc_arg, typval_T *argv, partial_T *partial, typval_T *rettv);
 void ex_disassemble(exarg_T *eap);
 int tv2bool(typval_T *tv);
+void emsg_using_string_as(typval_T *tv, int as_number);
 int check_not_string(typval_T *tv);
 /* vim: set ft=c : */