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/ext-attribute-no-value-error.patch

@@ -1,11 +0,0 @@
---- WebIDL.py
-+++ WebIDL.py
-@@ -3490,7 +3490,7 @@ class IDLBuiltinType(IDLType):
-                                       [self.location, attribute.location])
-                 assert not self.nullable()
-                 if not attribute.hasValue():
--                    raise WebIDLError("[TreatNullAs] must take an identifier argument"
-+                    raise WebIDLError("[TreatNullAs] must take an identifier argument",
-                                       [attribute.location])
-                 value = attribute.value()
-                 if value != 'EmptyString':