Fixes #18037

• Add CupertinoTheme using InheritedModel
• Remove material from cupertino gallery demos and use cupertino scaffolds
• Collect fonts and styles from various Cupertino widgets to the theme
• Refer to the them from various Cupertino widgets
• Move CupertinoThemeData to a model where derivation of dependent properties are done at read time rather than construction time
• Let the CupertinoThemeData's copyWith etc allow unspecified downstream properties be re-derived when upstream overrides change
• Add a new constructor to CupertinoButton to use the color of the theme instead of any particular color or null which means no color
• Add a dark mode to most Cupertino widgets
• Let Cupertino widgets demos in the gallery also respect the top level Material dark mode toggle
• There are no AnimatedCupertinoTheme or lerping because iOS is very non-visual-mutating-animation styled
• Create a MaterialBasedCupertinoThemeData that derives upwards from Material ThemeData rather than iOS defaults when upstream is unspecified
• MaterialBasedCupertinoThemeData is also based on InheritedModels where dependency is only created on individual properties read
• Material Theme also creates a CupertinoTheme and all Cupertino widgets only have dependencies on CupertinoTheme
• MaterialBasedCupertinoThemeData can also be copiedWith (and reused in a CupertinoTheme) and only specified properties rather than Material or iOS based derivations
• Let InheritedModel.inheritFrom not crash when no InheritedModels exist in the ancestry tree
• There's a single input API (CupertinoThemeData constructor) and a single output API regardless of if it's constructed and read right away (in which case no dependencies get created) or read via inheritance from CupertinoTheme or Material Theme. This is probably the most debatable point.

Upgrade Flow

Starting this PR, Cupertino widgets under a Material Theme will somewhat adapt to that Material Theme (such as the primaryColor, brightness etc).

To avoid Material adaptations and to let Cupertino widgets use a default blank CupertinoTheme, insert a CupertinoTheme under the Material Theme such as via

Material(
  data: ...,
  child: CupertinoTheme(
    data: ...,
    child: child,
  ),
);

or

MaterialApp(
  theme: ...,
  builder: (BuildContext context, Widget child) {
    return CupertinoTheme(
      data: ...,
      child: child,
    );
  },
  home: ...,
);

in case the Material Theme comes from the MaterialApp.