feat(core): add `Deno.core.loadExtScript()` for lazy-loaded scripts (#33739)

Adds `Deno.core.loadExtScript(specifier)` — a new API for loading
extension scripts on demand without going through V8's ES module infrastructure.

**Motivation:** Benchmarking showed that ES modules add ~31% snapshot
size overhead and ~4x slower serialization/deserialization compared to plain
scripts.