fix(git): add vendor in .gitignore

vendor/google.golang.org/protobuf/encoding/protojson/doc.go

@@ -1,11 +0,0 @@
-// Copyright 2019 The Go Authors. All rights reserved.
-// Use of this source code is governed by a BSD-style
-// license that can be found in the LICENSE file.
-
-// Package protojson marshals and unmarshals protocol buffer messages as JSON
-// format. It follows the guide at
-// https://developers.google.com/protocol-buffers/docs/proto3#json.
-//
-// This package produces a different output than the standard "encoding/json"
-// package, which does not operate correctly on protocol buffer messages.
-package protojson