[compiler] Closures capture let loop variables by final value, not per iteration

Closures capture let loop variables by final value, not per iteration. Standard TypeScript gives each for (let i ...) iteration a fresh binding; static TypeScript shares one binding, so every closure sees the loop's final value.

Repro: https://makecode.com/_1m7JkiYRK5Yw

const fns: (() => number)[] = []
for (let i = 0; i < 4; i++) {
    fns.push(() => i)
}
console.log(fns.map(f => f()).join(","))
// standard TypeScript: 0,1,2,3
// static TypeScript:   4,4,4,4

Confirmed on the simulator backend and on a micro:bit V2; both backends agree, so this is emitter-level scoping, not a backend bug. Workaround: capture through a function parameter (function mk(v: number) { return () => v }), which binds per call.

Start by running the linked MakeCode repro and compare the closure results with standard TypeScript. Trace the emitter-level scoping for for (let i ...) loops; done means each generated closure observes its iteration's value, with the simulator and micro:bit V2 behavior agreeing.