Support expressions for line-dasharray

[mvarendorff]

Currently the paint_factory only allows fixed arrays of numbers for line-dasharray and not expressions like ["get", "stroke-dasharray"]:

dart-vector-tile-renderer/lib/src/themes/paint_factory.dart

Lines 119 to 126 in 1d9f5f5

	final dashJson = paint['$prefix-dasharray'];
	if (dashJson != null && dashJson is List<num> && dashJson.length >= 2) {
	if (dashJson.any((element) => element < .0)) {
	logger.warn(() => '$prefix-dasharray contains value < 0');
	} else {
	dashArray = dashJson.map((e) => e.toDouble()).toList(growable: false);
	}
	}

[greensopinion]

feel free to send a PR!