update .npmignore for "third_party"

Based on a PR authored by Nolan Lawson: ChromeDevTools#160

The NPM package doesn't need the root_third party (~450MB),
however front_end/third_party now contains meaningful files,
so they're included.

Change-Id: I90cf20aa9b7bcced38be76b68fb4cc301bbcc1c8
Reviewed-on: https://chromium-review.googlesource.com/c/devtools/devtools-frontend/+/2342461
Auto-Submit: Paul Irish <[email protected]>
Reviewed-by: Mathias Bynens <[email protected]>
Commit-Queue: Mathias Bynens <[email protected]>

Author: paulirish

.npmignore

@@ -7,7 +7,11 @@ PRESUBMIT.py
 OWNERS
 ENG_REVIEW_OWNERS
 
-third_party
 test
 htaccess
 v8/include
+
+/out
+
+# Exclude the root third_party, but not front_end/third_party, etc
+/third_party