vim9: Rewrite highlighter in Vim9 Script

This is the one script, that is usually causing the most slow down.
Converting it to Vim9 Script should keep vim responsive and the users
happy. Hopefully that works now.

autoload/airline/msdos.vim

@@ -41,19 +41,43 @@ let s:basic16 = [
   \ [ 0xFF, 0xFF, 0xFF ]
   \ ]
 
-function! airline#msdos#round_msdos_colors(rgblist)
-  " Check for values from MSDOS 16 color terminal
-  let best = []
-  let min  = 100000
-  let list = s:basic16
-  for value in list
-    let t = abs(value[0] - a:rgblist[0]) +
-          \ abs(value[1] - a:rgblist[1]) +
-          \ abs(value[2] - a:rgblist[2])
-    if min > t
-      let min = t
-      let best = value
-    endif
-  endfor
-  return index(s:basic16, best)
-endfunction
+if !exists(":def") || (exists(":def") && get(g:, "airline_experimental", 0) == 0)
+
+	function! airline#msdos#round_msdos_colors(rgblist)
+		" Check for values from MSDOS 16 color terminal
+		let best = []
+		let min  = 100000
+		let list = s:basic16
+		for value in list
+			let t = abs(value[0] - a:rgblist[0]) +
+						\ abs(value[1] - a:rgblist[1]) +
+						\ abs(value[2] - a:rgblist[2])
+			if min > t
+				let min = t
+				let best = value
+			endif
+		endfor
+		return index(s:basic16, best)
+	endfunction
+
+	finish
+
+else
+
+  def airline#msdos#round_msdos_colors(rgblist: list<number>): string
+    # Check for values from MSDOS 16 color terminal
+    var best = []
+    var min  = 100000
+    var t = 0
+    for value in s:basic16
+      t = abs(value[0] - rgblist[0]) +
+          abs(value[1] - rgblist[1]) +
+          abs(value[2] - rgblist[2])
+      if min > t
+        min = t
+        best = value
+      endif
+    endfor
+    return string(index(s:basic16, best))
+  enddef
+endif