Ability to define which themes of a thematicStyling layer to be on from start

[Grammostola]

I have received feedback that it would be useful to be able to configure which themes of a thematicStyling layer should be active on map start.

Describe the solution you'd like

{
"thematicStyling": {
  "onAtStart": ["fluvial", "volcanic"]
  }
}

where "fluvial" is the rule name of a WMS style or the label of a vector style. For vector styles the "id" would fit better for the same reason as in #1962 but I think that similarly both should work here.

thematicStyling being parsable as both an object and a boolean: both true and an object would be considered truthy.

Describe alternatives you've considered
A second prop on the same level as thematicStyling, like thematicStylingPreset.