patch 9.1.0396: filetype: jj files are not recognized

Problem:  jj files are not recognized
Solution: recognize '*.jjdescription' files as jj filetype
          (Gregory Anders)

See: https://github.com/martinvonz/jj

closes: #14733

Signed-off-by: Gregory Anders <greg@gpanders.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/runtime/ftplugin/jj.vim b/runtime/ftplugin/jj.vim
new file mode 100644
index 0000000..cc5d700
--- /dev/null
+++ b/runtime/ftplugin/jj.vim
@@ -0,0 +1,19 @@
+" Vim filetype plugin
+" Language:	jj description
+" Maintainer:	Gregory Anders <greg@gpanders.com>
+" Last Change:	2024 May 8
+
+if exists('b:did_ftplugin')
+  finish
+endif
+let b:did_ftplugin = 1
+
+" Use the same formatoptions and textwidth as the gitcommit ftplugin
+setlocal nomodeline formatoptions+=tl textwidth=72
+setlocal formatoptions-=c formatoptions-=r formatoptions-=o formatoptions-=q formatoptions+=n
+setlocal formatlistpat=^\\s*\\d\\+[\\]:.)}]\\s\\+\\\|^\\s*[-*+]\\s\\+
+
+setlocal comments=b:JJ:
+setlocal commentstring=JJ:\ %s
+
+let b:undo_ftplugin = 'setl modeline< formatoptions< textwidth< formatlistpat< comments< commentstring<'