Function isRegistered

isRegistered(extensionId): boolean
JMap.Extension.isRegistered

Tell if an extension has been registered or not.

It can be useful to know if a JMap extension is in use or not.
Parameters
- extensionId: string
  The extension id
Returns boolean
Throws
Error if extensionId format is not correct

Example
```
// returns true if the JMap Document extension is in use or not
JMap.Extension.isRegistered("Document")
```