[split-view] track widths as percentages

[MikeMatusz]

Describe the feature:
Currently, you can specify size as a percentage, but it gets converted to a pixel width and when resizing panels, the width is always set in pixels. If the window is resized, for example, instead of keeping the same relative size of the panels, the absolute width of the resizable panel in pixels is maintained, and any non-resizable panels are the only ones affected. In some cases, this might be desirable, but in many it is not.

I'm proposing, that if the size attribute is specified as a percentage, it should continue to be tracked as a percentage. When resized, the pixel size would be converted into a percentage of the parent width or height depending on orientation, and the percentage value would be used for layout instead of pixels.

Describe any alternatives you've considered:
My current workaround involves handling the forge-split-view-panel-resize event, and updating the size attribute to the new calculated percentage, then calling the internal panel._foundation._applySize() method on window resize events.

Additional context: