add a basic QueryBuilder, test and doc

[Saibo-creator]

Add a QueryBuilder class to construct queries with support for decoders, prompts, models, and conditions.

Addressing #286

[lbeurerkellner]

This is really cool, thanks. Two small proposals for changes:

• Can we expose QueryBuilder as lmql.QueryBuilder and move the implementation to lmql/api? I would consider it externally facing API.

• Do we want to add convenient methods async run() and run_sync() directly on the QueryBuilder class so one can directly build + run it?

[lbeurerkellner]

Let's merge this :)