Implement read/write/deferred transaction modes

[CodingDoug]

Similar to what's been implemented in TS by @honzasp , batch and transaction should have a mandatory first argument to indicate the mode.

See: https://github.com/libsql/libsql-client-ts/blob/main/src/api.ts

As a temporary compatibility, the signature of the batch and transaction methods should continue to work as they have without a mode, defaulting to a write transaction.