Accept trailing comma on field extension list.

Right now trailing commas on field extensions are considered to be a missing identifiers.

Re-produce as:

 protoc \
  <(echo 'syntax = "proto3";\npackage test;\nimport "google/protobuf/descriptor.proto";\nextend google.protobuf.FieldOptions {\n  int32 a = 50000;\n  int32 b = 50001;\n}\nmessage Test {\n  int32 test = 1 [(a) = 1, (b) = 2,];\n}') \ 
  --cpp_out $(mktemp -d) \
  --proto_path /proc/self/fd/

(tested on 3.6.1)

Removing the comma at the end of (b) = 2, allows it to compile.

What language does this apply to?

Proto3 (presumably the same in proto2)

Describe the problem you are trying to solve.

I'd like to be able to supply trailing commas.

Describe the solution you'd like

Trailing commas are allowed.