Support return type assertions for steps

[acuarica]

When enabling type assertions in steps, e.g.

type Top<E extends Expr> = State<Inst, Expr> &{stack: {top: E}};

    CREATE: [0xf0, function create(state: State<Inst, Expr>): asserts state is Top<Create> {
    // [...]
    }

their usage step.CREATE(state); raises the following error

Assertions require every name in the call target to be declared with an explicit type annotation. ts(2775)

It may seem related to

• Error when assertion function calls aren't CFA'd microsoft/TypeScript#33622 and
• CFA Assertions Not Active on Implicitly-Typed Function Calls microsoft/TypeScript#33580

But not sure which name missing an explicit type annotation is referring to.