fix: allow for an optional semicolon where there is an optional comma in parseOptionValue

[jeffory-orrok]

Fixes #1569.

This is to conform with behavior found in text_format.cc

    // For historical reasons, fields may optionally be separated by commas or
    // semicolons.
    TryConsume(";") || TryConsume(",");

[alexander-fenster]

Thanks @jeffory-orrok!

Just to clarify, we are talking about https://github.com/protocolbuffers/protobuf/blob/master/src/google/protobuf/text_format.cc#L601-L603 (exact lines).

[jeffory-orrok]

Yes, and the same lines are repeated at https://github.com/protocolbuffers/protobuf/blob/master/src/google/protobuf/text_format.cc#L642-L644