Allow setting exception return value in exceptions

As far as I could tell the API does not currently allow setting the exception return value (EXC_RETURN) in exception handlers, which would be useful for implementing context switching etc.

Could we add an extra case to the #[exception] macro to also accept something like:

#[exception]
fn SysTick(exc: ExceptionReturn) -> ExceptionReturn

Where:

pub enum ExceptionReturn {
    HandlerMsp,
    HandlerFpuMsp,
    ThreadMsp,
    ....
}

This enum can then be used under the hood to set the correct value for LR/PC upon exception return.

I'd be happy to further experiment with this idea and turn it into a pull request.

Start by locating the #[exception] macro and its current handler forms, then review how EXC_RETURN is represented and used in Cortex-M exception return paths. Define the ExceptionReturn variants and the accepted handler signature, and verify that the macro can use the returned value to set the correct exception-return state for context switching.