[cupertino] Expose useful typography constants

### Use case

Similar to https://github.com/flutter/flutter/issues/179828

Several typography-related constants were privately defined in cupertino/menu_anchor.dart, but it may be useful to expose them for other widgets to access.

### Proposal

Rename and expose the following constants in cupertino/text_theme.dart:

```dart
/// The font family for menu items at smaller text scales.
const String _kBodyFont = 'CupertinoSystemText';

/// The font family for menu items at larger text scales.
const String _kDisplayFont = 'CupertinoSystemDisplay';

/// Base font size used for text-scaling calculations.
///
/// On iOS the text scale changes in increments of 1/17 (≈5.88%), as
/// observed on the iOS 18.5 simulator. Each step (1/17 of the base font size)
/// is referred to as one "unit" in the documentation for [CupertinoMenuAnchor]
const double _kCupertinoMobileBaseFontSize = 17.0;

/// The CupertinoMenuAnchor layout policy changes depending on whether the user is using
/// a "regular" font size vs a "large" font size. This is a spectrum. There are
/// many "regular" font sizes and many "large" font sizes. But depending on which
/// policy is currently being used, a menu is laid out differently.
///
/// Empirically, the jump from one policy to the other occurs at the following text
/// scale factors:
/// * Max "regular" scale factor ≈ 23/17 ≈ 1.352... (normalized text scale: 6)
/// * Min "accessible" scale factor ≈ 28/17 ≈ 1.647... (normalized text scale: 11)
///
/// The following constant represents a division in text scale factor beyond which
/// we want to change how the menu is laid out.
///
/// This explanation was ported from cupertino/dialog.dart.
const double _kMinimumAccessibleNormalizedTextScale = 11;

/// The minimum normalized text scale factor supported on iOS.
const double _kMinimumTextScaleFactor = 1 - 3 / _kCupertinoMobileBaseFontSize;

/// The minimum normalized text scale factor supported on iOS.
const double _kMaximumTextScaleFactor = 1 + 36 / _kCupertinoMobileBaseFontSize;
```

Alternatively, these constants could be placed in cupertino/constants.dart.

Start by comparing the private typography constants in cupertino/menu_anchor.dart with the public API organization in cupertino/text_theme.dart. Check whether cupertino/constants.dart is a better location, then expose the named constants with their documented values and verify that the resulting public API matches the proposal.

Written by the indexing model from the issue text.