fix(material/schematics): always add a custom theme with ng add

[andrewseguin]

ng add no longer provides options to include the prebuilt themes. Instead, always add a custom theme with user-specified palettes

The prompt is still present as it is today, except without the "custom" option, so users are still picking between four pairs of palettes as seen on material.angular.dev

// Include theming for Angular Material with `mat.theme()`.
// This Sass mixin will define CSS variables that are used for styling Angular Material
// components according to the Material 3 design spec.
// Learn more about theming and how to use it for your application's
// custom components at https://material.angular.dev/guide/theming
@use '@angular/material' as mat;

html {
  @include mat.theme((
    color: (
      primary: mat.$azure-palette,
      tertiary: mat.$blue-palette,
    ),
    typography: Roboto,
    density: 0,
  ));

  // Default the application to a light color theme. This can be changed to
  // `dark` to enable the dark color theme, or to `light dark` to defer to the
  // user's system settings.
  color-scheme: light;

  // Set a default background, font and text colors for the application using
  // Angular Material's system-level CSS variables. Learn more about these
  // variables at https://material.angular.dev/guide/system-variables
  background-color: var(--mat-sys-surface);
  color: var(--mat-sys-on-surface);
  font: var(--mat-sys-body-medium);
}

Also, remove the option to insert typography styles. Users should just use the system variables defined in the theming docs