TypeScript Error with loading Class in toastOptions and UX Suggestion for Loading Toast

[angelocala94]

I've encountered a couple of issues with the Sonner component that I'd like to bring to your attention:

1. TypeScript Error with loading Property: Using the className loading in toastOptions correctly applies the style, but TypeScript shows an error. This seems to be a mismatch in TypeScript definitions.
   

   sonner/src/types.ts

   Line 18 in 7560716

   loader?: string;

   <Sonner
      toastOptions={{
        // ... other options ...
        classNames: {
          // ... other class names ...
          loading:  'class-1 class-2',
        },
      }}
      // ... other props ...
    />

2. UX Suggestion for Loading Toast: Currently, the toast of type loading behaves like other toast types in terms of auto-dismissal. For a better user experience, I suggest that the loading toast should not auto-dismiss like the others. Instead, it would be more practical for it to remain visible indefinitely until manually dismissed using its ID.

[emilkowalski]

Great suggestions, added in #298