feat(list-key-manager): add support for horizontal directions

[crisbeto]

Updates the ListKeyManager to add support for using ltr and rtl directions. This will allow us to switch over even more components to the key manager (e.g. chips, tabs, stepper) as well as support more use cases (e.g. moving selection with the left/right arrows on a closed select).

[jelbourn]

What do you think of making this two method: withVerticalOrientation() and withHorizontalOrientation(dir)?

What did you have in mind for cases when it could have both turned on?

[crisbeto]

API-wise I'm fine with having separate methods, but it would make it slightly less convenient to deal with internally.

Regarding use cases, off the top of my head there are a couple:

1. Based on the a11y guidelines, horizontal tabs navigate using left/right arrows, whereas vertical tabs (which we use for the stepper) use both left/right and up/down.
2. Native select elements allow you to change the value when closed both by pressing the up/down arrows, as well as left/right.

[jelbourn]

Hmmmm.........

My main reason or not liking everything in one method is that it lets users provide an invalid combination (even though it's caught). Another option would be to separate the orientation from the direction into separate methods, so you'd have something like

withOrientation(...orientations: Horizontal | Vertical) { }

withDirectionality(dir) { } // defaults to LTR

[crisbeto]

Reworked based on the feedback @jelbourn.

[jelbourn]

LGTM

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