runtime(js): fix useOptionalChain

[smorimoto]

It's already widely used in the runtime and can be unlocked since it's already included in the ECMASCRIPT documentation.

PR Overview

This pull request updates the codebase to use ECMAScript optional chaining for cleaner, more concise property and method access.

• Replaces several conditional property checks (e.g. "x && x.prop") with optional chaining (e.g. "x?.prop").
• Applies the optional chaining update consistently across multiple runtime files, improving code readability.

Reviewed Changes

File	Description
runtime/js/dynlink.js	Applies optional chaining for symbol lookup in the dynamic linker.
runtime/js/fs_fake.js	Uses optional chaining when extracting parent directory from regex match.
runtime/js/obj.js	Replaces traditional checks with optional chaining in custom type tests.
runtime/js/sys.js	Uses optional chaining for process and child process properties.
runtime/js/jslib.js	Updates loops to use optional chaining for trampoline execution.
runtime/js/fs.js	Applies optional chaining when matching drive roots.
runtime/js/hash.js	Uses optional chaining in custom hash computation functions.
runtime/js/effect.js	Replaces loop condition checks with optional chaining.
runtime/js/compare.js	Uses optional chaining for custom comparison functions.
runtime/wasm/runtime.js	Applies optional chaining for process event handler setup.

Copilot reviewed 11 out of 11 changed files in this pull request and generated no comments.