[@mantine/core] fix portal props typings and override order

[jvdsande]

Current implementation of portalProps requires children to be specified, when we don't want or need them here. There is also an override order that is wrong if withinPortal is passed both to portalProps and the component props itself.

For TypeScript users:

• children is now removed from the prop types everywhere
• withinPortal is removed from the prop types when it exists on the component type (which is always the case for now I think)
• target is removed from the prop types if it's overwritten inside the component implementation

For JavaScript users:

• withinPortal and target passed through portalProps do not override internal target and component-level withinPortal anymore

[rtivital]

Thanks!