[Request] Support login via GitHub authentication provider

Since VSCode v1.48 it is possible to use existing accounts to authenticate your extension. Microsoft & GitHub account providers are generally available for all users.

See more details: https://code.visualstudio.com/updates/v1_48#_using-authentication-providers

Since hackmd.io supports login via GitHub it would be fitting to avail the same in this extension.

However this request does not mean to completely remove/replace the existing Email/Password login.

PS: My reason for this request is that I want to use this extension but do not want to enter email/password while the VSCode's Authentication Provider API seems to be using the underlying OS's method of choice to store/retrieve credentials which in most cases is more secure by default.