patch 9.0.1885: Vim9: no support for abstract methods

Problem:  Vim9: no support for abstract methods
Solution: Add support for defining abstract methods in an abstract class

closes: #13044
closes: #13046

Signed-off-by: Christian Brabandt <cb@256bit.org>
Co-authored-by: Yegappan Lakshmanan <yegappan@yahoo.com>
diff --git a/src/errors.h b/src/errors.h
index fc40fe8..5776906 100644
--- a/src/errors.h
+++ b/src/errors.h
@@ -3495,6 +3495,12 @@
 	INIT(= N_("E1369: Duplicate member: %s"));
 EXTERN char e_cannot_define_new_function_as_static[]
 	INIT(= N_("E1370: Cannot define a \"new\" function as static"));
+EXTERN char e_abstract_must_be_followed_by_def_or_static[]
+	INIT(= N_("E1371: Abstract must be followed by \"def\" or \"static\""));
+EXTERN char e_abstract_method_in_concrete_class[]
+	INIT(= N_("E1372: Abstract method \"%s\" cannot be defined in a concrete class"));
+EXTERN char e_abstract_method_str_not_found[]
+	INIT(= N_("E1373: Abstract method \"%s\" is not implemented"));
 EXTERN char e_cannot_mix_positional_and_non_positional_str[]
 	INIT(= N_("E1400: Cannot mix positional and non-positional arguments: %s"));
 EXTERN char e_fmt_arg_nr_unused_str[]