LocaleProperty does not set its type.

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Discovered while working on secondLocaleProperty: LocaleProperty in phetsims/number-pairs#22 ...

In localeProperty.ts, LocaleProperty is exported so that clients can create other instances. But unlike other Property subclasses, LocaleProperty does not set its phetioValueType or valueType, and leaves that to clients. So I'm getting this error in Number Pairs:

Assertion failed: PhET-iO Properties must specify a phetioValueType: numberPairs.global.model.preferences.secondLocaleProperty

The singleton localeProperty is instantiated like this:

const localeProperty = new LocaleProperty( phet.chipper.locale, {
  tandem: Tandem.GENERAL_MODEL.createTandem( 'localeProperty' ),
  valueType: 'string',
  phetioFeatured: true,
  phetioValueType: StringIO,
  phetioDocumentation: 'Specifies language currently displayed in the simulation'
} );

So this should be a matter of moving valueType: 'string' and phetioValueType: StringIO to a LocaleProperty constructor (it currently has a default a constructor) and defining LocalePropertyOptions.

Discussed with @zepumph in Slack#developer, and the above sounds correct and desirable, so I'll go for it.

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... So this should be a matter of moving valueType: 'string' and phetioValueType: StringIO to a LocaleProperty constructor (it currently has a default a constructor) and defining LocalePropertyOptions.

... which is exactly what I did in 4810d1d.

@zepumph would you please review? Close if OK.

[zepumph]

Perfect. Thank you for doing this.