Add canonical .txtpb to ProtobufText extensions

[sergiitk]

Per https://protobuf.dev/reference/protobuf/textformat-spec/#text-format-files:

.txtpb is the canonical text format file extension and should be preferred to the alternatives. This suffix is preferred for its brevity and consistency with the official wire-format file extension .binpb. The legacy canonical extension .textproto still has widespread usage and tooling support. Some tooling also supports the legacy extensions .textpb and .pbtxt. All other extensions besides the above are strongly discouraged; in particular, extensions such as .protoascii wrongly imply that text format is ascii-only, and others like .pb.txt are not recognized by common tooling.