Add `PrimaryButton` and `SecondaryButton`

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,
  }
);

The issue names no files, tests, or entry points. First locate the template's existing shared widget and styling conventions, then review how the requested PrimaryButton and SecondaryButton constructors fit them. Done means both button variants support the listed default, custom, and child APIs with enabled and loading states.