Enhancement: Show back button on first page

[guyluz11]

Is your feature request related to a problem? Please describe.
I am using the introduction screen in the middle of the app because of the functionality that it gives.
Not just as introduction screen.
The first introduction page does not show the back button arrow.

Describe the solution you'd like
Setting showBackButton to true will show the button even in the first page.

Describe alternatives you've considered

Additional context
Maybe we can add another propraty show back button in the first page.

Package version or git hash

• introduction_screen: ^3.1.14