Add Voicing to NumberPicker

[zepumph]

Since this extends AccessibleValueHandler (and therefore voicing), it isn't clear how related this will be to phetsims/sun#730.

It will support phetsims/ratio-and-proportion#441 though.

[zepumph]

Implemented above. @jessegreenberg this was pretty straightforward, would you mind reviewing and seeing if GFLB still looks good to you.

[terracoda]

@zepumph, I just noticed that Mass Control number pickers on master are no longer getting context responses when context responses are checked in preferences. In the published version, GFLB's Number Pickers do get context responses, so maybe some changes here need a second look?

[zepumph]

@terracoda, I made phetsims/gravity-force-lab-basics#320 to fix the issues I made in GFLB. Anything else to change in common code for NumberPicker?

[terracoda]

Oops, missed this issue. I'll review the work very soon.

[terracoda]

The Mass number pickers in GFLB and the Left and Right Value number pickers in RaP are working well, and seem to be working in the exact same way as well.

The reading blocks are different, but that is intentional. The Mass Control reading blocks in GFLB are not keyboard accessible (this is intentional) whereas the Current Challenge reading block is keyboard accessible (this is intentional).

[terracoda]

Assuming I can close this issue as Voicing for NumberPicker seems very nice.