NumberControl: support for accessibleName and helpText, display units in A11y View

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Related to phetsims/models-of-the-hydrogen-atom#67 and phetsims/sun#901 ...

NumberControl does not support accessibleName and helpText. You can provide these options, but they are ignored.

Developers have been using this undesirable workaround in sim-specific code:

const numberControl = new NumberControl( ..., {
  ...
  sliderOptions: {
    ...
    accessibleName: 'The accessible name.',
    helpText: 'The help text.'
  },
} );

Besides the obvious problems of reaching inside NumberControl to access its Slider, this workaround does not address the units associated with the NumberControl's value. NumberControl is responsible for units, Slider has no knowledge of units. So for example in MOTHA, we end up with this in the A11y View, where we want to see "380 nm".