webpacker.yml: correctly ignore node_modules (#1863)

/node_modules/ should be parsed to a js regex, however js-yaml parses it as a
string. As a result, files under the /node_modules/ directory get watched. This
commit switches to using an anymatch pattern which works as expected.

lib/install/config/webpacker.yml

@@ -72,7 +72,7 @@ development:
     headers:
       'Access-Control-Allow-Origin': '*'
     watch_options:
-      ignored: /node_modules/
+      ignored: '**/node_modules/**'
 
 
 test: