fix(line): correct pointSymbol datum type to match runtime data

[restareaByWeezy]

Closes #2818

The pointSymbol prop was typed as DotsItemSymbolComponent<Point<Series>>, but at runtime the datum passed to the symbol component is actually Point<Series>['data'] (i.e. the raw data item), not the full Point object. You can see this in Points.tsx where datum: point.data is mapped before passing to DotsItem.

Changed the type to DotsItemSymbolComponent<Point<Series>['data']> so datum.stalled (or whatever custom fields are on the data) is correctly typed without needing datum.data.stalled.

Also removed the as any cast on the symbol prop in Points.tsx since the types now line up.