Create a top-level "third_party" directory

This directory now contains third_party software that is vendored into
the Servo source tree. The idea is that it would eventually hold
webrender and other crates from mozilla-central as well with a standard
patch management approach for each.

components/script/dom/bindings/codegen/parser/debug.patch

@@ -1,10 +0,0 @@
---- WebIDL.py
-+++ WebIDL.py
-@@ -8827,6 +8827,7 @@ class Parser(Tokenizer):
-                 module=self,
-                 outputdir=outputdir,
-                 errorlog=logger,
-+                debug=False,
-                 write_tables=False,
-                 # Pickling the grammar is a speedup in
-                 # some cases (older Python?) but a