patch 8.2.0149: maintaining a Vim9 branch separately is more work

Problem:    Maintaining a Vim9 branch separately is more work.
Solution:   Merge the Vim9 script changes.
diff --git a/runtime/doc/Makefile b/runtime/doc/Makefile
index 514a012..65dc843 100644
--- a/runtime/doc/Makefile
+++ b/runtime/doc/Makefile
@@ -149,6 +149,7 @@
 	version7.txt \
 	version8.txt \
 	vi_diff.txt \
+	vim9.txt \
 	visual.txt \
 	windows.txt \
 	workshop.txt
@@ -289,6 +290,7 @@
 	version8.html \
 	vi_diff.html \
 	vimindex.html \
+	vim9.html \
 	visual.html \
 	windows.html \
 	workshop.html