Serialize addon coordinates are backwards internally

[Tyriar]

This is the wrong way around:

xterm.js/addons/addon-serialize/src/SerializeAddon.ts

Lines 448 to 451 in 49eab6b

	return handler.serialize({
	start: { x: selection.start.y, y: selection.start.x },
	end: { x: selection.end.y, y: selection.end.x }
	});

xterm.js/addons/addon-serialize/src/SerializeAddon.ts

Lines 30 to 33 in 49eab6b

	const startRow = range.start.x;
	const endRow = range.end.x;
	const startColumn = range.start.y;
	const endColumn = range.end.y;

I don't think this is an API facing bug, just confusing code.