Add `PrimaryButton` and `SecondaryButton`

[nivisi]

These are pretty common and we recreate them almost on every project. Would be cool to have them in-place and to design them after we create a project from the template.

The most common parameters would be:

• String text
• VoidCallback onPressed
• bool isEnabled
• bool isLoading

As most of the time we will use preconfigured button sizes, paddings etc, we could create a named constructor .custom to be able to configure the following parameters:

• double paddingVertical
• double paddingHorizontal
• Color backgroundColor
• Color foregroundColor
• TextStyle textStyle

And also a constructor for a custom child, if we ever need to use sth like an icon, e.g. withChild

So the interface is the following. Same for SecondaryButton.

final defaultButton = PrimaryButton(
    String text, {
    VoidCallback? onPressed,
    bool isEnabled = true,
    bool isLoading = false,
  }
);

final defaultButtonWithChild = PrimaryButton.withChild(
  {
    VoidCallback? onPressed,
    bool isEnabled = true,
    bool isLoading = false,
    required Widget child,
  }
);

final customButton = PrimaryButton.custom(
    String text, {
    VoidCallback? onPressed,
    bool isEnabled = true,
    bool isLoading = false,
    TextStyle? textStyle,
    double paddingVertical = 4.0, // or whatever
    double paddingHorizontal = 12.0, // or whatever
    Color? backgroundColor, // in the build method, we will be able to use this custom color
    Color? foregroundColor,  // in the build method, we will be able to use this custom color
  }
);

final customButtonWithChild = PrimaryButton.customWithChild(
  {
    VoidCallback? onPressed,
    bool isEnabled = true,
    bool isLoading = false,
    TextStyle? textStyle,
    double paddingVertical = 4.0, // or whatever
    double paddingHorizontal = 12.0, // or whatever
    Color? backgroundColor, // in the build method, we will be able to use this custom color
    Color? foregroundColor, // in the build method, we will be able to use this custom color
    required Widget child,
  }
);