patch 9.1.1647: filetype: Cangjie files are not recognized

Problem:  filetype: Cangjie files are not recognized
Solution: Detect *.cj files as cangjie filetype, include a syntax plugin
          (WuJunkai2004)

This commit introduces a new syntax highlighting file for the Cangjie
programming language, includes 4 parts as required:
- The main syntax file: runtime/syntax/cangjie.vim
- The filetype detection rule in: runtime/filetype.vim
- The documentation update in: runtime/doc/syntax.txt
- Some menus

References:
- https://gitcode.com/Cangjie
- https://cangjie-lang.cn/

fixes: 18014
closes: #18027

Signed-off-by: WuJunkai2004 <wujunkai20041123@outlook.com>
Signed-off-by: Doug Kearns <dougkearns@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>

runtime/filetype.vim

@@ -2433,6 +2433,9 @@ au BufNewFile,BufRead *.il,*.ils,*.cdf		setf skill
 " Cadence
 au BufNewFile,BufRead *.cdc			setf cdc
 
+" Cangjie
+au BufNewFile,BufRead *.cj			setf cangjie
+
 " SLRN
 au BufNewFile,BufRead .slrnrc			setf slrnrc
 au BufNewFile,BufRead *.score			setf slrnsc