Fix bug with indentation of nested parent rows

[mollykreis]

Pull Request

🤨 Rationale

This fixes a bug I found where the indentation of nested parent rows can sometimes be incorrect, particularly when the order of rows changes (such as when sorting a column) in a way that changes the nesting level that is set on a nimble-table-row element that is getting reused during a binding update.

The problematic binding was this condition in the table row's template: x.isParentRow && x.nestingLevel > 0. That statement is volatile, but it cannot be marked as such in the template. Therefore, I moved the evaluation into a getter named isNestedParent that could be marked as @volatile.

👩‍💻 Implementation

See Rationale section above

🧪 Testing

Manually tested in storybook with a problematic set of data that I found the problem with

✅ Checklist

• I have updated the project documentation to reflect my changes or determined no changes are needed.

[mollykreis]

@atmgrifter00, will you buddy this PR for me?