textVariableAnchor style for SymbolLayer works on iOS

[MichaelDanielTom]

Fixes #3105 on iOS

[mfazekas]

@MichaelDanielTom thanks much, looks great.
In future it can be generalised so we can reuse for all array of enums like mglStyleValueArrayTextWritingMode

maps/ios/RNMBX/RNMBXStyleValue.swift

Lines 413 to 434 in 73272d4

	func mglStyleValueEnum<Enum : RawRepresentable>() -> Value<Enum> where Enum.RawValue == String {
	if let value = value as? Dictionary<String,Any> {
	let value = RNMBXStyleValue.convert(value["stylevalue"] as! [String:Any])
	if let value = value as? String {
	return Value.constant(Enum(rawValue: value )!)
	} else if let value = value as? [Any] {
	if let parsedExpression = (logged("mglStyleValueEnum: failed to parse expression") {
	try parseExpression(value)
	}) {
	return .expression(parsedExpression)
	} else {
	return Value.constant(Enum(rawValue: value[0] as! String)!)
	}
	} else {
	Logger.log(level: .error, message:"Invalid value for enum (nor string, not array): \(value) returning something")
	return Value.constant(Enum(rawValue: value as! String)!)
	}
	} else {
	Logger.log(level: .error, message:"Invalid value for enum: \(value) returning something")
	return Value.constant(Enum(rawValue: value as! String)!)
	}
	}

[MichaelDanielTom]

@mfazekas just force pushed something that changed map to compactMap - was building properly in my project with the former but not building with the example project. Using compactMap works with the example project too. Not sure if the difference was a swift version, caching issue, or something else.