vim-patch:9.0.2062: Janet files are not recognised

Problem:  Janet files are not recognised
Solution: Add filename and shebang detection (without
          adding an extra filetype plugin)

Those are used by the Janet language:
http://www.janet-lang.org

closes: vim/vim#13400

c038427d2a

Co-authored-by: Doug Kearns <dougkearns@gmail.com>

runtime/lua/vim/filetype.lua

@@ -536,6 +536,7 @@ local extension = {
   jal = 'jal',
   jpr = 'jam',
   jpl = 'jam',
+  janet = 'janet',
   jav = 'java',
   java = 'java',
   jj = 'javacc',

runtime/lua/vim/filetype/detect.lua

@@ -1652,8 +1652,9 @@ local patterns_hashbang = {
   ['icon\\>'] = { 'icon', { vim_regex = true } },
   guile = 'scheme',
   ['nix%-shell'] = 'nix',
-  ['crystal\\>'] = { 'crystal', { vim_regex = true } },
+  ['^crystal\\>'] = { 'crystal', { vim_regex = true } },
   ['^\\%(rexx\\|regina\\)\\>'] = { 'rexx', { vim_regex = true } },
+  ['^janet\\>'] = { 'janet', { vim_regex = true } },
 }
 
 ---@private