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/synmenu.vim

@@ -110,6 +110,7 @@ an 50.20.110 &Syntax.C.C++ :cal SetSyn("cpp")<CR>
 an 50.20.120 &Syntax.C.C# :cal SetSyn("cs")<CR>
 an 50.20.130 &Syntax.C.Cabal\ Haskell\ build\ file :cal SetSyn("cabal")<CR>
 an 50.20.140 &Syntax.C.Calendar :cal SetSyn("calendar")<CR>
+an 50.20.140 &Syntax.C.Cangjie :cal SetSyn("cangjie")<CR>
 an 50.20.150 &Syntax.C.Cascading\ Style\ Sheets :cal SetSyn("css")<CR>
 an 50.20.160 &Syntax.C.CDL :cal SetSyn("cdl")<CR>
 an 50.20.170 &Syntax.C.Cdrdao\ TOC :cal SetSyn("cdrtoc")<CR>