[ALOY-812] Alloy: Automatically convert string-values for number-properties like borderWidth in TSS

When doing an app for both iOS and Android, I usually set the default unit to dp. This works great, but some properties like borderWidth and borderRadius are always interpreted as px on Android. So for the right size, I need to use Alloy's builtin measurement.dpToPx. However, TSS does not (longer) support using methods, only (global) values. This means for every different value I use, I need to pre-calculate the px-value in alloy.js and store it in Alloy.Globals so I can use it in the TSS. This is far from ideal. Why not add builtin casting of non-number values to number-values for properties that require that type? For example, in TSS I'd have:

"View": {
  borderWidth: '5dp'
}

Which then would be compiled to:

borderWidth: Alloy.measurement.toNumber('5dp');

The Alloy.measurement.toNumber method then should read the unit from the string and perform the appropriate conversion to get the number-value (px).