Provide a rectangle getArea implementation for horizontal bars

[etimberg]

Resolves #4480

It should be noted that since #5857 getArea() is not used anywhere in the code.

~/D/o/chartjs ❯❯❯ grep getArea -rni src/*
src/elements/element.arc.js:68:	getArea: function() {
src/elements/element.point.js:55:	getArea: function() {
src/elements/element.rectangle.js:207:	getArea: function() {
~/D/o/chartjs ❯❯❯ 

I considered removing the method from the different elements but that would be a breaking change. I don't think anyone is using it, but we can't guarantee it.

[benmccann]

Should we mark the method as deprecated as well?

[kurkle]

I'm not sure this should be deprecated, its probably needed at some point anyway.
As a side note (v3 in mind): width and height should be "right", so this is kind of wrong place to check orientation.

[simonbrunel]

That's the kind of methods we would have preferred private.

[benmccann]

Looks like you'll need to rebase this against master after #6039 is merged to get Travis to pass

[kurkle]

Disclaimer: this has little or nothing to do with this PR

For V3 we should think of removing orientation from rectangle and adding a bar element that brings orientation, start and end properties.

I would also brainstorm more around the properties, we should be able to extract at least configurable and those that should be transitioned. We should also try not to have to check for hasOwnProperty in interpolate (so would have to make sure those transitioning properties always exist = move the check out of the loop).