[protobuf2] The applied occurrence of `syntax` should not be mandatory.

[kaby76]

In protobuf2, the syntax for proto has syntax as mandatory.

grammars-v4/protobuf/protobuf2/Protobuf2.g4

Lines 16 to 18 in 6b51773

	proto
	: syntax (importStatement | packageStatement | optionStatement | topLevelDef | emptyStatement_)* EOF
	;

This is wrong. The Protobuf2 spec says: proto = [syntax] { import | package | option | topLevelDef | emptyStatement } (https://protobuf.dev/reference/protobuf/proto3-spec/#proto_file).

Consequently, it fails to parse the first example in the Extensions section in the programming guide (https://protobuf.dev/programming-guides/proto2/#ext-example).

// file kittens/video_ext.proto

import "kittens/video.proto";
import "media/user_content.proto";

package kittens;

// This extension allows kitten videos in a media.UserContent message.
extend media.UserContent {
  // Video is a message imported from kittens/video.proto
  repeated Video kitten_videos = 126;
}

I mentioned this problem in another comment, #4629 (comment), chasing down another problem.