Components consuming load functions no longer need non-null assertions

[spiffytech]

Currently, load functions are clumsy to use from TypeScript because the RouteSectionProps type marks the data prop as optional. If you try working around that in your component (such as Component<Required<RouteSectionProps<MyProps>>>), you instead get compile errors that your component's props type isn't assignable to RouteDefinition.

The only workaround I've found is runtime null checks, or type-level non-null assertions, inside every component that uses load functions.

This PR marks the data prop as non-optional: the route props will always have a data field, set to the RouteSectionProps generic parameter type. If not load function is supplied, the field's value would be undefined, with a type of unknown.

What happens if the route doesn't have a load function defined? Your component would be declared as Component<RouteSectionProps>. Without the type param supplied, the data prop receives type unknown, and is unusable inside your component.

If a component sometimes does/doesn't receive load data, it can use Component<RouteSectionProps<Foo | undefined>>.

This approach provides accurate types when you have a load function, and still signals the data prop cannot be read when you do not have a load function.

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Latest commit: e963ad3a0bd414d0788801ba853b3d395237a438

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