Updated minor syntax, to allow for TypeScript compiler to be happier

[kachihro]

Description

Have logged an issue, and then investigated within source code ;
#713

Fix #1 : to return a specific type, Promise, which then includes a reference in IMPORTS - to avoid an 'include' statement within TYPINGS

Fix #2 : no need for additional reference (= USER) - this causes a 'missing comma' error in TypeScript compiler (!?!)

Testing

No functionality change, simply to provide for more succinct and precise code - when compiling with TypeScript 2.2.2
(This is needed for a SharePoint SPFX webpart being created)

Checklist

• [ X ] I have added documentation for new/changed functionality in this PR or in auth0.com/docs
  (No changed functionality)

• [ X ] All active GitHub checks for tests, formatting, and security are passing

• [ X ] The correct base branch is being used, if not master

[lgtm-com]

This pull request introduces 1 alert when merging edb7390 into be9e9d7 - view on LGTM.com

new alerts:

• 1 for Unused variable, import, function or class

[frederikprijck]

Thanks for the PR, looks like this introduces an unused import so we might want to fix that so that the code is actually using the imported IdToken.