Keep the stack order when the first dataset of a stack is hidden

[exclusiveabhi]

Fixes #11978

When the first dataset of a stack is hidden (for example from the legend), the stacks of a grouped stacked bar chart swap places. In the issue, hiding "Comp 1" moves stack 1 to the left of stack 0, even though "Comp 2" still has visible bars in stack 0.

_getStacks() builds the stack list in the order the stacks are first seen among the visible datasets only. With the first dataset of stack 0 hidden, stack 1 is seen first and takes the first position.

Hidden datasets are now part of that walk, but only to hold the position of a stack that still has visible bars, so the stacks keep their order. A stack with no visible bars still takes no space (#6368), and stacked: false charts are unchanged. The dataset lookup moved to a private getGroupedMetas() helper.

Added a test that hides the first dataset with chart.hide(0) and checks the stack indexes and the bar positions. It fails on master.

[exclusiveabhi]

@LeeLenaleee @etimberg could one of you approve the CI run when you get a chance? This fixes #11978, where hiding the first dataset of a stack from the legend reorders the stacks. Happy to adjust anything.