Optimize bigint handling

[stephenh]

Currently when using forceLong=bigint, protobuf.js still produces longs, and we convert to/from bigints by going through strings.

In theory we should optimize this to not use strings, by either leveraging some code from the longfn fork:

https://github.com/consento-org/longfn/blob/main/index.mjs#L654

https://github.com/consento-org/longfn/blob/main/index.mjs#L100

Or just waiting until protobufjs natively supports bigints.