Add yarn (#3432)

* add yarn.lock

* fix comment

* remove yarn test

* add test

* fix test

* try fix again

* try 3rd time

* check filename and firstline for yarn lockfile

lib/linguist/generated.rb

@@ -80,7 +80,8 @@ module Linguist
       generated_jflex? ||
       generated_grammarkit? ||
       generated_roxygen2? ||
-      generated_jison?
+      generated_jison? ||
+      generated_yarn_lock?
     end
 
     # Internal: Is the blob an Xcode file?
@@ -479,5 +480,13 @@ module Linguist
       return lines[0].start_with?("/* parser generated by jison ") ||
              lines[0].start_with?("/* generated by jison-lex ")
     end
+
+    # Internal: Is the blob a generated yarn lockfile?
+    #
+    # Returns true or false.
+    def generated_yarn_lock?
+      return false unless name.match(/yarn\.lock/)
+      return lines[0].include?("# THIS IS AN AUTOGENERATED FILE")
+    end
   end
 end