[enhancement] support for gpb text format

[sebastiw]

Hi,

I'm wondering about the support for *.textproto files, i.e. text-format for google protobufs. https://protobuf.dev/reference/protobuf/textformat-spec/ As far as I can see gpb doesn't support it, but maybe I'm mistaken?

Otherwise I've made a scanner and parser for the format, https://github.com/sebastiw/gpb_text and am wondering if you are interested in merging this code into gpb.

In future I also want to add validation using the proto-definition.

Best regards