Add printnow builtin for eager output

[patx]

Motivation

• Provide a way to eagerly evaluate arguments and print immediately instead of returning a Deferred for print when arguments contain Thunk/Deferred/Task values.
• Make it easy to force immediate output in the async/no-await model used by the evaluator.

Description

• Recognize printnow calls in the evaluator by adding is_printnow_ident in Expr::Call and resolve its arguments with resolve_if_needed instead of resolve_deferred_only when building the call argument list.
• Add a new stdlib builtin printnow in install_stdlib that eagerly resolves Thunk, Deferred, and Task argument values and prints their repr() immediately, returning None.
• Update the evaluator argument-handling logic so spawn still receives raw/unresolved values, print keeps its deferred behavior, and printnow forces eager resolution.
• Document printnow(...) usage in README.md alongside the async expression statement behavior.

Testing

• No automated tests were run for this change.

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Codex Task