Feature: Go-like `(val, ok)` handler deduction for `select`

[mzfhhhh]

Title: Feature: Go-like (val, ok) handler deduction for select

Motivation

Currently, consuming data inside a select handler requires manually checking channel state and popping:

select(ch, {
    if (!ch.ready) { /* closed */ }
    else {
        auto val = ch.front;
        ch.popFront;
    }
});

This is verbose and error-prone. It would be great to mimic Go's val, ok := <-ch mental model, which is widely understood and very LLM-friendly.

Proposal

Utilize D's compile-time reflection to deduce the handler's signature.
1. (val, ok) style (Safe and explicit):

select(ch, (int val, bool ok) {
    if (ok) { 
        // Process val 
    } else { 
        // Channel closed and drained 
    }
});

2. val only style (Convenient, val is T.init if closed):

select(ch, (int val) {
    // Process val (might be T.init if closed)
});

This significantly improves DX and aligns perfectly with standard concurrency mental models.