moonbit: lay groundwork for async runtime support

[peter-jerry-ye]

This is the first step in the MoonBit async work, and is intended to keep the later changes easier to review.

The diff includes a few preparatory refactors and language/runtime updates:

• add separate MoonBit Debug support via --derive-debug without changing the existing --derive-show behavior
• switch export-side generated stubs over to declare, so bindgen no longer needs to emit an export stub.mbt
• update the MoonBit test/build flow to the current moon build layout and accept both _build/... and target/... output locations
• land the initial async runtime scaffolding and associated generator refactors that later PRs will build on

I kept this as a foundation PR on purpose so the follow-up async behavior changes can be reviewed separately.

Validation:

• cargo check -p wit-bindgen-moonbit
• cargo run test --languages rust,moonbit tests/runtime --artifacts target/artifacts --rust-wit-bindgen-path ./crates/guest-rust --runner wasmtime