Factor out class NumberModelTypeControl.

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In NumberPairsPreferencesNode.ts, the code below should be factored out into class NumberModelTypeControl extends PreferencesControl. That will make it easier to address the PhET-iO instrumentation of this component, which is currently lacking/incorrect -- the control itself currently has no tandem and no visibleProperty, and numberModelRadioButtonGroup should be a child of that control.

    const numberModelRadioButtonGroup = new RectangularRadioButtonGroup( NumberPairsConstants.NUMBER_MODEL_TYPE_PROPERTY,
      [
        {
          createNode: () => new Text( 'Number Bond', PreferencesDialogConstants.CONTROL_LABEL_OPTIONS ),
          value: 'numberBondModel',
          tandemName: 'numberBondModelRadioButton'
        },
        {
          createNode: () => new Text( 'Bar Model', PreferencesDialogConstants.CONTROL_LABEL_OPTIONS ),
          value: 'barModel',
          tandemName: 'barModelRadioButton'
        }
      ], {
        orientation: 'horizontal',
        tandem: options.tandem.createTandem( 'numberModelRadioButtonGroup' ),
        isDisposable: false,

        // Hide or show the entire row, not just the radio button
        phetioVisiblePropertyInstrumented: false
      } );
    const numberModelControl = new PreferencesControl( {
      labelNode: new Text( 'Number Model Type', PreferencesDialogConstants.CONTROL_LABEL_OPTIONS ),
      controlNode: numberModelRadioButtonGroup
    } );

This will also clean up NumberPairsPreferencesNode.ts, which is already becoming a little difficult to read due to lack of whitespace between unrelated chunks of code:

    const numberModelRadioButtonGroup = new RectangularRadioButtonGroup( NumberPairsConstants.NUMBER_MODEL_TYPE_PROPERTY,
...
      } );
    const numberModelControl = new PreferencesControl( {
      labelNode: new Text( 'Number Model Type', PreferencesDialogConstants.CONTROL_LABEL_OPTIONS ),
      controlNode: numberModelRadioButtonGroup
    } );
    const secondLanguageControl = new SecondLanguageControl(
...
      } );

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Other things related to the implementation of this feature:

• NumberModelType should be created.
• NumberPairsConstants.NUMBER_MODEL_TYPE_PROPERTY should be a StringEnumerationProperty<NumberModelType>, not Property<string>.
• Replace literals 'Number Bond' and 'Bar Model' with LocalizedStringProperty instances.

Rename to correspond to the Property type and UI labeling ("Number Model Type"):

• const numberModelRadioButtonGroup => numberModelTypeRadioButtonGroup
• const numberModelControl => numberModelTypeControl
• tandem 'numberModelRadioButtonGroup' => 'numberModelTypeRadioButtonGroup'