fix: [#285] Use Implicit Tiled layer order to default z index

[eonarheim]

Closes #285

This PR implements implicit z-ordering based on the Tiled editor order. Z-index can still be overridden with a custom property if desired. The first layer will start at -1 z-index (the previous default) and will increment based on the Tiled order.

Example of inserted tiles using the implicit z-index, no more custom property!

Small-ish breaking change that moves all the optional properties to an option bag in the constructor

// New constructor
const map = new TiledMapResource('path/to/map.tmx', { firstLayerZIndex: -2 });

// Old constructor
const map = new TiledMapResource('path/to/map.tmx', undefined, -2);

TODOs before merge:

• Tests
• @kamranayub @jedeen Feedback on the ctor?

[kamranayub]

Oh yeah I prefer the new constructor for sure

[kamranayub]

Looks pretty good. Left some suggestions to improve clarity and making property handling consistent for z index values.

Feedback addressed

[kamranayub]

I won't block on these suggestions but I do think they make the code intent clear.

[kamranayub]

Are the other class members here aligned correctly?

[kamranayub]

Here too

[kamranayub]

I get this is a clever way to coerce but it's not clear what this does at first glance especially for someone not as familiar with hacks in JS.

Might as well use the standard, it's available in all browsers we support and it's clearer.

Suggested change

	resultLayer.id = +layer.id;
	resultLayer.id = Number.parseInt(layer.id);

Note radix is optional and is 10 by default anyway!

Note also that there was code before that didn't coerce, because this one character can get lost in the noise.

[kamranayub]

Suggested change

	// coerce to integer
	return +finalZ
	return Number.parseInt(finalZ);

😁