Add outlineWidth property to FillLayer

[abouroubi]

New Feature

When we create a FillLayer, we have the option to choose the outlineColor, but we can't specify the outlineWidth.

I know that there is a LineLayer class, that has a lineWidth property, and with some hacks we can use it as an outline for the FillLayer. First, performance wise it's not optimal when we have a huge data set, and second in my case it's not even possible, because the LineLayer only accepts Line or MultiLine, while my data is a list of Polygons and is loaded from a VectorSource, so I can't transform them to MultiLines to use this workaround.

Why

Being able to have a thicker outline for accessibility reason.