Update isExpoPushToken type signature (#36)

* Update isExpoPushToken type signature

I think this is a better fit for what the function actually does: we don't know the type of the thing coming in, but we do know the type if it returns true.

https://www.typescriptlang.org/docs/handbook/advanced-types.html#using-type-predicates

src/ExpoClient.ts

@@ -51,7 +51,7 @@ export class Expo {
   /**
    * Returns `true` if the token is an Expo push token
    */
-  static isExpoPushToken(token: ExpoPushToken): boolean {
+  static isExpoPushToken(token: unknown): token is ExpoPushToken {
     return (
       typeof token === 'string' &&
       (((token.startsWith('ExponentPushToken[') || token.startsWith('ExpoPushToken[')) &&