Fix identifying transaction statements

[MasterOdin]

PR fixes identifying BEGIN TRANSACTION statements. A number of SQL dialects (e.g. bigquery, sqlite) use BEGIN both as a block opener, and as keyword right before TRANSACTION to start a transaction (or in the case of sqlite, there's also three types of transactions one can use, see docs). As such, we now pass in the nextToken when parsing in addition to the current token, allowing us to peek ahead one and see if BEGIN acts as a block opener, or to begin a transaction.

I debated on making BEGIN check only for those dialects that use this explicitly, but decided against it and to make it generic in that there's probably no client that has BEGIN TRANSACTION and expects that to be operating in block mode as a fully working query.