feat: Adds support for custom keyboard pages and other customizations

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.

None

[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.