Add subcommand to CLI

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Currently the prql binary has no subcommands — it always writes SQL (or AST depending on the arguments). This limits us from doing other things in the future, like querying a DB, or opening a console like PyPrql does.

Let's add a subcommand like write-sql, even if it's initially the only one at first. (Open to other names, though having all subcommands be verbs is nice for consistency).