Add highlight directive to the rST reader (#6140)

test/rst-reader.rst

@@ -107,6 +107,30 @@ And:
    def my_function(x):
        return x + 1
 
+If we use the highlight directive, we can specify a default language
+for literate blocks.
+
+.. highlight:: haskell
+
+::
+
+  -- this code is in haskell
+  data Tree = Leaf | Node Tree Tree
+
+::
+
+  -- this code is in haskell too
+  data Nat = Zero | Succ Nat
+
+.. highlight:: javascript
+
+::
+
+  -- this code is in javascript
+  let f = (x, y) => x + y
+
+.. highlight::
+
 Lists
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