feat: Adds support for custom keyboard pages and other customizations

[Sub6Resources]

Description

• BREAKING: bool secondPage has been replaced with int page in MathFieldEditingController to allow for more than two pages.
• Adds support for custom keyboard pages using the new customPages attribute that takes a list of CustomButtonPages.
  • Each CustomButtonPage takes a List<List<KeyboardButtonConfig>> as well as an optional label or icon to be shown using the PageButtonConfig.
  • The existing pages were wrapped in this class.
• Adds a fontSize attribute to MathKeyboard and a keyboardButtonFontSize attribute to MathField to allow the font size of the keyboard buttons to be adjusted.
• Adds a heightFactor attribute to KeyboardButtonConfig to allow for customization of the height of keyboard rows.
• Adds widget tests for MathKeyboard to test the functionality of custom and default pages on the keyboard.

Related issues & PRs

None

Checklist

• [x] I have made myself familiar with the CaTeX contributing guide.
• [x] I added a PR description.
• [x] I linked all related issues and PRs I could find (no links if there are none).
• [x] If this PR changes anything about the main math_keyboard or example package (also README etc.), I created an entry in CHANGELOG.md (## UPCOMING RELEASE if the change on its own is not worth an update).
• [x] If this PR includes a notable change in the math_keyboard package, I updated the version according to Dart's semantic versioning.
• [x] If there is new functionality in code, I added tests covering all my additions.
• [x] All required checks pass.

[absar]

@Sub6Resources @edhom this is a very useful PR can it be merged

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