Support more types in Result<(), E> for #[wasm_bindgen(start)]

[daxpedda]

Motivation

Reducing code duplication in cross-platform code-bases by getting as close as possible to what fn main() supports.
I would argue that #2994 and #2970 are also cross-platform issues, because error libraries are usually not targeted towards wasm-bindgen but all sorts of targets.

See #3076 and #3252 for more fn main() compatibility.

Proposed Solution

Make #[wasm_bindgen(start)] as compatible as possible with Termination, which is what is expected in fn main() by Rust.

Currently the return error type has to be a JsValue or (), but very commonly error types like anyhow::Error are used.
We should still continue supporting JsValue at the same time.

Alternatives

I don't know of any alternative for wasm-bindgen to apply here. This can always be solved on the user side with more #[cfg(...)]s.