feat(slidesInView): separation of concerns + added method to public api

[pimdewit]

I thought slidesInView would be a nice addition to the public API, it is also a nice opportunity for separation of concerns. I extracted the snippet from _updateSlidesA11y and added it to its own getter.

[coveralls]

Pull Request Test Coverage Report for Build 68

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 31 unchanged lines in 1 file lost coverage.
• Overall coverage decreased (-0.7%) to 90.214%

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Files with Coverage Reduction	New Missed Lines	%
dist/macro-carousel-test.js	31	90.21%

Totals
Change from base Build 62:	-0.7%
Covered Lines:	539
Relevant Lines:	578

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💛 - Coveralls

[ciampo]

• needs more tests (as per my comment)
• we should consider refactoring other parts of the code that kind of perform the same calculation (like in the slidesStatusCaption function and in the attributeChanged function
• we should add docs for this property (and specify it's read-only) (actually, because there is no setter, what happens when writing the value of the property?)

[ciampo]

We need more tests:

• with different values of selected (not just 0)
• with different values of slidesPerView (definitely at least 1 as well)

And then we need to test for edge cases:

• what if selected is set on purpose out of range? (e.g. there are 5 slides, and we set selected to 7)
• what if slidesPerView is set to 0?
• what if looping is enabled, and the selected slide is the last? slidesInView should be [last, first...]

Any anything else that comes to mind