refactor: replace runtime module options with wazero ModuleConfig

[stephenafamo]

This migrates runtime module-instantiation customization from several ad hoc qjs.Option fields to wazero.ModuleConfig.

Instead of exposing separate CWD, StartFunctionName, Stdout, and Stderr fields, qjs.Option now exposes a single ModuleConfig field for advanced module setup.

When ModuleConfig is nil, qjs now creates a default wazero module config that:

• clears wazero's default start functions with WithStartFunctions()
• mounts the current working directory at /
• enables walltime, nanotime, and nanosleep
• wires stdout/stderr to the host process

This preserves the previous behavior of qjs.New() after the refactor.
Without explicitly clearing start functions, bare wazero.NewModuleConfig() would run _start during instantiation and break normal runtime creation.

[stephenafamo]

Implements #45