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>

runtime/syntax/jj.vim

@@ -0,0 +1,20 @@
+" Vim syntax file
+" Language:	jj description
+" Maintainer:	Gregory Anders <greg@gpanders.com>
+" Last Change:	2024 May 8
+
+if exists('b:current_syntax')
+  finish
+endif
+let b:current_syntax = 'jj'
+
+syn match jjAdded "A .*" contained
+syn match jjRemoved "D .*" contained
+syn match jjChanged "M .*" contained
+
+syn region jjComment start="^JJ: " end="$" contains=jjAdded,jjRemoved,jjChanged
+
+hi def link jjComment Comment
+hi def link jjAdded Added
+hi def link jjRemoved Removed
+hi def link jjChanged Changed