protoparse: trying to set extension field on a custom option whose type is message results in syntax error

[bufdev]

https://github.com/protocolbuffers/protobuf/blob/master/src/google/protobuf/unittest_custom_options.proto#L281

src/google/protobuf/unittest_custom_options.proto:281:29:syntax error: unexpected '('
src/google/protobuf/unittest_custom_options.proto:282:29:syntax error: unexpected '('
src/google/protobuf/unittest_custom_options.proto:282:53:syntax error: unexpected ')'

It seems that the double parentheses are tripping up protoreflect

[jhump]

Welp, I thought I had implemented support for this correctly. I see the intent to support this in the grammar.

But I think there is a legit shift-reduce conflict in the grammar; there are actually many such conflicts reported by yacc, but I thought they were all harmless and that the generated parser would still work. In this case, I think the conflict causes the parser to not properly recognize this production. Luckily, I think it's a pretty easy fix.

[jhump]

Fixed in #316