patch 8.2.3965: Vim9: no easy way to check if Vim9 script is supported

Problem:    Vim9: no easy way to check if Vim9 script is supported.
Solution:   Add has('vim9script').
diff --git a/src/evalfunc.c b/src/evalfunc.c
index 9b49d81..d61797a 100644
--- a/src/evalfunc.c
+++ b/src/evalfunc.c
@@ -5929,6 +5929,7 @@
 		0
 #endif
 		},
+	{"vim9script", 1},
 	{"vimscript-1", 1},
 	{"vimscript-2", 1},
 	{"vimscript-3", 1},