[Chart] tooltip slot prop type error

[techniq]

Currently if you pass the tooltip prop to <Chart> this will wrap the <slot> in a <TooltipContext> which we then use to pass the current value of $tooltip as a slot prop. This is used in a lot of cases including this Area example as well as when in manual mode, such as for Geo, Pie, etc.

ex.

<Chart tooltip let:tooltip>
  ...
</Chart>

Typescript appears to not be able to infer this and throws an error, so we need to find a way to explicitly define this.