vim9: finish early for legacy Vims

fixes #2382

autoload/airline/extensions/tabline.vim

@@ -265,10 +265,8 @@ if !exists(":def") || (exists(":def") && get(g:, "airline_experimental", 0) == 0
     endif
     return group
   endfunction
-
+  finish
 else
-
-  " Vim9 Script implementation
   def s:toggle_off(): void
     airline#extensions#tabline#autoshow#off()
     airline#extensions#tabline#tabs#off()

autoload/airline/extensions/tabline/formatters/default.vim

@@ -43,6 +43,7 @@ if !exists(":def") || (exists(":def") && get(g:, "airline_experimental", 0) == 0
     endif
     return _
   endfunction
+  finish
 else
   " Vim9 Script implementation
   def airline#extensions#tabline#formatters#default#format(bufnr: number, buffers: list<number>): string

autoload/airline/themes.vim

@@ -80,6 +80,7 @@ if !exists(":def") || (exists(":def") && get(g:, "airline_experimental", 0) == 0
       let a:palette.accents.purple = [ '#af00df' , '' , 128 , '' ]
     endif
   endfunction
+  finish
 else
   " New Vim9 Script Implementation
   def airline#themes#generate_color_map(sect1: list<any>, sect2: list<any>, sect3: list<any>): dict<any>