[Lens] Enable fitting functions for stacked area charts

[flash1293]

In the latest version of elastic-charts, stacked charts support fitting functions as well https://github.com/elastic/elastic-charts/blob/master/CHANGELOG.md#features

Currently, fitting functions are disabled for stacked area charts.

By enabling it, we can use the same fitting functions for them as well.

[elasticmachine]

Pinging @elastic/kibana-app (Team:KibanaApp)

[flash1293]

@nickofthyme You linked the eui update to this issue - is it actually fixed by the new elastic-charts version alone? Looks like this also needs a small change within Lens (enabling the option in the menu)

[nickofthyme]

@flash1293 Yeah there was a dependency on eui for some unrelated chart changes but specifically @elastic/charts@21.0.0 was what enabled stacked fit functions.

I added the stacked_area type to the fit function enablement logic, not sure if there is anything else to do than that. But with the change in #75700 I was able to use fit functions on stacked areas.