Refactor tooltip formatter from function to class

[nreese]

The current tooltip formatter implementation contains a lot of nested functions that return functions. Ultimately, the last returned function is used as the formatter to format the event into an html string. [PR embed visualization in tooltip] (#14102) adds a cleanUp function to the tooltipFormatter.

@thomasneirynck suggested the following updates that were deemed to large for the scope of PR 14102 but still valuable.

• It would be nice to refactor tooltipFormatProvider's to return a class instead of a function.
  something like:

class TooltipFormatter {
   constructor(parentVis) {
   }
  
   formatTooltipContents(event) {
   }
  
   cleanUp(){
  }
   
}

• Have the formatter return a dom element instead of an html string.

[nreese]

There has been a lot of refactoring and tooltipFormatProvider no longer exists and this suggestion is no longer useful