fix(#4003): add custom loader component via slot

[jose-fernandez]

Description

This PR #4003 adds custom loader components on va-inner-loading via slot.

Also it adds a new example in storybook for custom loader. Default and custom loaders share the same loading trigger button.

Extra Notes

The name of the slot is loading as vaInfiniteScroll also contains a loader slot and it is named loading. For consistency, I´ve chosen the same name. Personally, I think a more explicit name would be loader as it is a loader component what we will pass through the slot. I would use loading more for state cases.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Improvement/refactoring (non-breaking change that doesn't add any feature but make things better)

[m0ksem]

I also like loader more, sadly we choose a different one in different component. Good noticed. Great job, thanks!