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/update.sh

@@ -1,13 +0,0 @@
-wget https://hg.mozilla.org/mozilla-central/raw-file/tip/dom/bindings/parser/WebIDL.py -O WebIDL.py
-patch < abstract.patch
-patch < debug.patch
-patch < callback-location.patch
-patch < union-typedef.patch
-patch < inline.patch
-patch < readable-stream.patch
-
-wget https://hg.mozilla.org/mozilla-central/archive/tip.zip/dom/bindings/parser/tests/ -O tests.zip
-rm -r tests
-mkdir tests
-unzip -d tests -j tests.zip
-rm tests.zip WebIDL.py.orig