Fixing "has/in" on variables proxy object

When using WA.state, using `"myVariable" in WA.state` would always return false.
This is now fixed by adding a "has" method on the Proxy class.

Also, added a `WA.state.hasVariable` method.

docs/maps/api-state.md

@@ -9,6 +9,7 @@ Moreover, `WA.state` functions can be used to persist this state across reloads.
 ```
 WA.state.saveVariable(key : string, data : unknown): void
 WA.state.loadVariable(key : string) : unknown
+WA.state.hasVariable(key : string) : boolean
 WA.state.onVariableChange(key : string).subscribe((data: unknown) => {}) : Subscription
 WA.state.[any property]: unknown
 ```