tools: Add trailing newlines to files that were missing them

This avoids things like:

  ~$ cat tools/validation_helpers.py
  ...
          return self.is_block(ast_node) and has_heading~$

with your next prompt way over to the right ;).

tools/chunk-options.R

@@ -23,4 +23,4 @@ hook_out <- function(x, options) {
 }
 
 knit_hooks$set(source = hook_in, output = hook_out, warning = hook_out,
-               error = hook_out, message = hook_out)
+               error = hook_out, message = hook_out)

tools/validation_helpers.py

@@ -239,4 +239,4 @@ def is_callout(self, ast_node):
         else:
             has_heading = False
 
-        return self.is_block(ast_node) and has_heading
+        return self.is_block(ast_node) and has_heading