Add a feature box selection example

This PR adds a "feature box selection" example. This example combines the use of a DragBox interaction and a Select interaction to select features by drawing boxes on the map.
Some linters enforce the use of `===` for comparing a variable to the value `0`.  I guess this is because `'' == 0`, `[] == 0`, and `[[]] == 0` evaluate to `true`.

In the case of `ends.length` using `==` would certainly be sufficient, but my editor's linter makes the line red, which makes my eyes bleed.

That being I'm fine with using `===` when comparing to a non-zero value as well. But we currently do not consistently favor one pattern (`==`) over the other (`===`) in the library.