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/abstract.patch

@@ -1,10 +0,0 @@
---- WebIDL.py
-+++ WebIDL.py
-@@ -1987,6 +1987,7 @@ class IDLInterface(IDLInterfaceOrNamespace):
-                 or identifier == "RunConstructorInCallerCompartment"
-                 or identifier == "WantsEventListenerHooks"
-                 or identifier == "Serializable"
-+                or identifier == "Abstract"
-             ):
-                 # Known extended attributes that do not take values
-                 if not attr.noArguments():