[@mantine/core] clearer typescript error for polymorphic components

[jvdsande]

As discussed on Discord, this PR changes the following structure:

C extends React.ElementType ? PolymorphicComponentProps<C, SomeProps>  : never

to

C extends React.ElementType ? PolymorphicComponentProps<C, SomeProps>  : (SomeProps & { component: React.ElementType })

This allows for clearer error messages when providing a wrong value as component: instead of having all props failing, now only the component prop fails.

Furthermore it moves the conditional typing logic inside PolymorphicComponentProps directly to avoid duplicating it everywhere.

Note: one use case was not discussed on Discord, it's using the "generic" structure:

<Avatar<wrong> size="sm" />

Then the error is a bit less straightfoward, because it starts by saying that component prop is missing while required, then when we change to:

<Avatar<wrong> size="sm" component="wrong" />

The error moves to "component" and says "Type '"wrong"' is not assignable to type 'ElementType'"

I'm not quite sure why the docs tells users to use this structure though since simply providing the component props makes TS infer correctly the other props too. Using the generic component instance structure is redundant in this situation.

[rtivital]

Thanks for PR!