Wrap std::sys::windows::process::Process's main_thread_handle with Option<T>.

[TYPEmber]

how about make main_thread_handle as Option?

Except creating a child process, it is difficult for us to get the main_thread_handle of another process. If we make this item Option, we could extend struct Process's use case. Such as building a process manager, which holds some process's handle without main_thread_handle.

pub struct Process {
    handle: Handle,
    main_thread_handle: Option<Handle>,
}

impl Process {
    ...

    pub fn main_thread_handle(&self) -> Option<BorrowedHandle<'_>> {
        self.main_thread_handle?.as_handle()
    }

    ...
}

Maybe related to #96723