Make Node#{pre,ap}pend_to return the appended node

This commit introduces a slight incompatibility, but hopefully
it's a worthy one.

The rationale is that I often tend to write code like:
`DOM { div }.append_to(other_node)` to construct a DOM tree and
append it in one go. When I try to capture the tree, I can't -
I think it makes no sense to return a parent.

Regarding other cases where this is done this way, I would note
jQuery even if some people may say it's an unpopular comparison,
but I want this library to give jQuery users a familiar interface.
Nokogiri has no say about this - it doesn't have {pre,ap}pend_to.

Do note, that it makes no change to << and >> which should
return a parent node.

Author: hmdne

opal/browser/dom/node.rb

@@ -165,6 +165,7 @@ def add_previous_sibling(node = nil, &block)
   # @param node [Node] the node to append to
   def append_to(node)
     node << self
+    self
   end
 
   # Get an array of ancestors.
@@ -396,6 +397,7 @@ def path
   # @param node [Node] the node to prepend to
   def prepend_to(node)
     node >> self
+    self
   end
 
   # @!attribute previous