worldedit selection helper function + improvements to error feedback

[H41ogen]

Closes #2816

[Argmaster]

I would prefer to return a struct here instead of a vector to keep things explicit:

const Selection = struct {
	minPos: Vec3i,
	maxPos: Vec3i,
}

Its a bit silly, but I think we should distinguish between that arbitrary pos1/pos2 and the normalized min/max positions.

[Argmaster]

Alternatively we could just change variable names in caller, but that struct will automatically enforce consistent names in callers.

[Argmaster]

Actually we don't need to min/max it - Blueprint.capture does that anyway no?

[Argmaster]

I would like this interface improved anyway tho; its a little bit out of scope, but it would be nice to change Blueprint.capture to accept Selection that has fn init(pos1, pos2) and that init min/maxes the positions.

src/blueprint.zig:77

pub fn capture(allocator: NeverFailingAllocator, selection: Selection) CaptureResult {

Selection struct should be placed inside blueprint.zig

const Selection = struct {
	minPos: Vec3i,
	maxPos: Vec3i,

	fn init(pos1, pos2) Selection {
		...	
	}
}

Then we can remove min/max-ing from capture

/// Remove:
		const startX = @min(pos1[0], pos2[0]);
		const startY = @min(pos1[1], pos2[1]);
		const startZ = @min(pos1[2], pos2[2]);

		const endX = @max(pos1[0], pos2[0]);
		const endY = @max(pos1[1], pos2[1]);
		const endZ = @max(pos1[2], pos2[2]);

And just go

const startX, const startY, const startZ = selection.minPos;
const endX , const endY , const endZ = selection.maxPos;

We can also add fn size to Selection and remove it from capture body (src/blueprint.zig:86)

[IntegratedQuantum]

I agree with @Argmaster: This should be a struct with min and max for better readability.
You don't need to do all the interface changes he suggested here, but at least the newly introduced function should return a struct.