import js variables with two types

[geotre]

`import std/jsffi type StyleSpecification = ref object of JsRoot LngLat = ref 
object of JsRoot MapOptions*[T: StyleSpecification | cstring] {.importjs.} = 
ref object of JsRoot container*: cstring center*: LngLat zoom*: int style*: T 
let opts = MapOptions[string]( container: "foo", center: LngLat(), zoom: 1, 
style: "bar" ) `

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