feat: Signal proto3 optional support

[yoheimuta]

ref. #234

I checked the support locally as below.

(base) ❯ cat test.proto
syntax = "proto3";

package test;

message Test {
  optional string foo = 1;
}
(base) ❯ protoc --protolint_out=. test.proto

(base) ❯ cat test3.proto
syntax = "proto3";

package test;

message testTest {
  optional string foo = 1;
}
(base) ❯ protoc --protolint_out=. test3.proto
[test3.proto:5:1] Message name "testTest" must be UpperCamelCase like "TestTest"
--protolint_out: protoc-gen-protolint: Plugin failed with status code 1.