patch 9.0.1820: Rexx files may not be recognised

Problem:  Rexx files may not be recognised
Solution: Add shebang detection and improve disambiguation of *.cls
	  files

closes: #12951

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

src/testdir/test_filetype.vim

@@ -945,6 +945,8 @@ def s:GetScriptChecks(): dict<list<list<string>>>
     forth:  [['#!/path/gforth']],
     icon:   [['#!/path/icon']],
     crystal: [['#!/path/crystal']],
+    rexx:   [['#!/path/rexx'],
+            ['#!/path/regina']],
   }
 enddef
 
@@ -2045,7 +2047,22 @@ func Test_cls_file()
 
   " Rexx
 
-  call writefile(['# rexx'], 'Xfile.cls')
+  call writefile(['#!/usr/bin/rexx'], 'Xfile.cls')
+  split Xfile.cls
+  call assert_equal('rexx', &filetype)
+  bwipe!
+
+  call writefile(['#!/usr/bin/regina'], 'Xfile.cls')
+  split Xfile.cls
+  call assert_equal('rexx', &filetype)
+  bwipe!
+
+  call writefile(['/* Comment */'], 'Xfile.cls')
+  split Xfile.cls
+  call assert_equal('rexx', &filetype)
+  bwipe!
+
+  call writefile(['::class Foo subclass Bar public'], 'Xfile.cls')
   split Xfile.cls
   call assert_equal('rexx', &filetype)
   bwipe!

src/version.c

@@ -699,6 +699,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1820,
 /**/
     1819,
 /**/