Markdown reader: case-insensitive alert keywords (#11836)

The keywords (e.g. `TIP`) for GFM-style alerts need not be all caps.
This commit fixes that. It also adds the class `alert` to the produced
Divs to allow for namespacing.

This commit changes the emitted classes from the alert extension
of the basic Markdown reader to add the class `alert` to the
produced DIVs to allow for namespacing.

test/command/9716.md

@@ -4,7 +4,7 @@
 > A suggestion.
 ^D
 [ Div
-    ( "" , [ "tip" ] , [] )
+    ( "" , [ "alert" , "tip" ] , [] )
     [ Div ( "" , [ "title" ] , [] ) [ Para [ Str "Tip" ] ]
     , Para [ Str "A" , Space , Str "suggestion." ]
     ]