perf: add fused callback info parts

[bartlomieju]

Summary

Adds FunctionCallbackInfo::get_parts() as a single callback-entry shim that returns the isolate pointer, return value, callback data, and argument length together.

This lets raw callbacks avoid separate C++ FFI calls for the common callback prologue:

• CallbackScope::new(info) / isolate lookup
• ReturnValue::from_function_callback_info(info)
• FunctionCallbackArguments::from_function_callback_info(info) for callback data and argument length

The new FunctionCallbackInfoParts can be passed directly to CallbackScope, and FunctionCallbackArguments::from_function_callback_info_parts(info, &parts) reuses the already-loaded callback data and length.

Deno usage

The main Deno consumer should be the generated slow op callback prologue in:

• libs/ops/op2/dispatch_slow.rs: with_scope, with_retval, and with_fn_args currently generate separate callback-info reads for slow ops.
• libs/ops/op2/dispatch_async.rs: async slow op generation reuses those same helpers and can benefit from the same callback-entry snapshot.

A smaller direct use is:

• libs/core/ops_metrics.rs: slow_metrics_dispatch currently constructs FunctionCallbackArguments only to read callback data and recover OpCtx; it can use info.get_parts().data directly.

There are also scattered core callbacks, for example libs/core/error.rs and libs/core/runtime/bindings.rs, but the op2 generator is the valuable centralized follow-up because it affects many generated slow callback paths.

Validation

• cargo fmt
• cargo check
• cargo check --benches

I also tried cargo test --test test_api function_callback_info_parts -- --nocapture; it compiled Rust code but failed at the native link step in my local tree because the existing native archive is missing _v8__FunctionCallbackInfo__GetParts plus pre-existing missing symbols _v8__Object__SetLazyDataProperty and _v8__String__Concat.