patch 8.2.4172: filetype detection for BASIC is not optimal

Problem:    Filetype detection for BASIC is not optimal.
Solution:   Improve BASIC filetype detection. (Doug Kearns)

runtime/autoload/dist/ft.vim

@@ -67,13 +67,29 @@ func dist#ft#FTasmsyntax()
   endif
 endfunc
 
-" Check if one of the first five lines contains "VB_Name".  In that case it is
-" probably a Visual Basic file.  Otherwise it's assumed to be "alt" filetype.
-func dist#ft#FTVB(alt)
-  if getline(1).getline(2).getline(3).getline(4).getline(5) =~? 'VB_Name\|Begin VB\.\(Form\|MDIForm\|UserControl\)'
+func dist#ft#FTbas()
+  if exists("g:filetype_bas")
+    exe "setf " . g:filetype_bas
+    return
+  endif
+
+  " most frequent FreeBASIC-specific keywords in distro files
+  let fb_keywords = '\c^\s*\%(extern\|var\|enum\|private\|scope\|union\|byref\|operator\|constructor\|delete\|namespace\|public\|property\|with\|destructor\|using\)\>\%(\s*[:=(]\)\@!'
+  let fb_preproc  = '\c^\s*\%(#\a\+\|option\s\+\%(byval\|dynamic\|escape\|\%(no\)\=gosub\|nokeyword\|private\|static\)\>\)'
+  let fb_comment  = "^\\s*/'"
+  " OPTION EXPLICIT, without the leading underscore, is common to many dialects
+  let qb64_preproc = '\c^\s*\%($\a\+\|option\s\+\%(_explicit\|_\=explicitarray\)\>\)'
+
+  let lines = getline(1, min([line("$"), 100]))
+
+  if match(lines, fb_preproc) > -1 || match(lines, fb_comment) > -1 || match(lines, fb_keywords) > -1
+    setf freebasic
+  elseif match(lines, qb64_preproc) > -1
+    setf qb64
+  elseif match(lines, '\cVB_Name\|Begin VB\.\(Form\|MDIForm\|UserControl\)') > -1
     setf vb
   else
-    exe "setf " . a:alt
+    setf basic
   endif
 endfunc