Fix 8.4 deprecations

[zuuperman]

Fixes deprecation errors that you receive when using php 8.4

Example:
ERROR: PhpSchool\CliMenu\MenuStyle::setBorder(): Implicitly marking parameter $colour as nullable is deprecated, the explicit nullable type must be used instead

[Copilot]

Pull Request Overview

This PR fixes PHP 8.4 deprecation warnings by explicitly marking nullable parameters with the ? type declaration syntax instead of relying on implicit nullability from default null values.

• Updates method signatures to use explicit nullable types (?string, ?int, ?Terminal, ?MenuStyle)
• Maintains backward compatibility while adhering to PHP 8.4 requirements
• Affects constructor parameters, method parameters with optional null defaults

Reviewed Changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
src/Util/ColourUtil.php	Updates validateColour method to use explicit nullable string parameter
src/MenuStyle.php	Updates constructor and multiple methods to use explicit nullable parameters
src/Dialogue/Dialogue.php	Updates write method to use explicit nullable int parameter
src/CliMenu.php	Updates constructor and dialogue methods to use explicit nullable parameters
src/Builder/SplitItemBuilder.php	Updates enableAutoShortcuts method to use explicit nullable string parameter
src/Builder/CliMenuBuilder.php	Updates constructor and multiple styling methods to use explicit nullable parameters

[AydinHassan]

Thanks @zuuperman !