Enable strict mode

[AlexElin]

Description

Enable strict TS options.
Set them to the same values that Angular sets during creating a new project via ng new.
I fixed new type errors + add generics for better type checking

References

https://blog.angular.io/angular-cli-strict-mode-c94ba5965f63
https://angular.io/guide/strict-mode

Checklist

• I have added documentation for new/changed functionality in this PR or in auth0.com/docs
• All active GitHub checks for tests, formatting, and security are passing
• The correct base branch is being used, if not master

[frederikprijck]

Thanks for this PR, we always appreciate contributions like this.
However, in general, it is a good idea to open an issue to discuss the subject first, to avoid doing work that might end up not being merged.

As this includes a breaking change, we will need to consider how and if we want to include this.
If we merge this PR, the following two code snippets, that would work fine in applications that use strict mode, will fail now:

config: AuthConfig;
constructor(private config: Auth0ClientConfig) {
  this.config = config.get(); // Strict Error: Type 'AuthConfig | undefined' is not assignable to type 'AuthConfig'
}

or

constructor(private config: Auth0ClientConfig) {
  const config = config.get();
  console.log(config.domain); // Strict Error: Object is possibly undefined
}

I am a big fan of using TypeScript strict mode, but if it is the only reason for introducing a breaking change, I am not sure I am keen on doing so.

I will keep this PR open, as I want to discuss this with our team as well. But keep in mind this isn't a priority for us at the moment, especially given the breaking change it introduces.

[AlexElin]

In AuthClientConfig I changed get(): AuthConfig -> get(): AuthConfig | undefined becasue config looked as optional field.
I could revert the signature change and impelment the method like

  get(): AuthConfig {
    return this.config as AuthConfig;
  }

This would not change method API. So, as I understand, it would not be a breaking change