Improvements to tooltip positioners (#9944)

* Improve positioner types; allow overriding xAlign and yAlign

* More type improvements; pass in Chart as third parameter

* Expose chart as part of TooltipModel

I initially passed the Chart element as the third parameter to the positioner; however, Scale and LegendElement elements expose `this.chart`, and sample code for positioners used `this._chart`, so documenting the chart member and giving it a public name seems to make more sense.

* Update documentation

* Fix documentation

* Fix issues from code review