Revert "Remove yarn.lock from auto-generated files list (follow-up) (#…

…4459)" (#4460)

This reverts commit 3bc8185.

lib/linguist/generated.rb

@@ -87,6 +87,7 @@ def generated?
       generated_roxygen2? ||
       generated_pkgdown? ||
       generated_jison? ||
+      generated_yarn_lock? ||
       generated_grpc_cpp? ||
       generated_dart? ||
       generated_perl_ppport_header? ||
@@ -539,6 +540,15 @@ def generated_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 false unless lines.count > 0
+      return lines[0].include?("# THIS IS AN AUTOGENERATED FILE")
+    end
+
     # Internal: Is this a protobuf/grpc-generated C++ file?
     #
     # A generated file contains: