New Alias System: Add the Alias class

[seisman]

This PR adds the Alias class. It's the basis of the new alias system (#4000) and class-like parameters (see #3995).

So, please review PRs #3995 and #4000 first.

[Copilot]

Pull Request Overview

This PR introduces the Alias class as a dataclass-based wrapper for the existing _to_string function to establish the foundation for a new alias system. The class provides a more structured and object-oriented approach to handling parameter aliasing between PyGMT's long-form arguments and GMT's short-form options.

• Adds the Alias dataclass with attributes matching _to_string parameters
• Implements a _value property that delegates to the existing _to_string function
• Provides comprehensive documentation with usage examples

Comments suppressed due to low confidence (1)

pygmt/alias.py:185

• [nitpick] The property name '_value' uses a leading underscore which typically indicates a private/internal attribute, but this appears to be the main interface for accessing the converted value. Consider using 'value' or 'converted_value' for better API clarity.

    def _value(self) -> str | list[str] | None: