Fixes #9

Supersedes #12

• Uses decorator pattern to wrap original PromptBuilder in Prompt_Builder, allowing to use "snake_case" method names instead of "camelCase" method names, as per WordPress Coding Standards.
• As an alternative flavor, Prompt_Builder_With_WP_Error extends Prompt_Builder and catches any exceptions thrown in the original PromptBuilder, turning them into WP_Error return values from the terminate methods.
  • Only the terminate methods return WP_Error objects, to maintain the fluent interface. As soon as an exception is caught in a chain of method calls, all subsequent calls become no-ops, and only the terminate method will return the WP_Error.
• A very basic AI_Client is included to streamline instantiation of the prompt builder classes. While it is somewhat equivalent to AiClient from the underlying PHP AI Client SDK, there is no need for parity at this point, as most methods on AiClient are for more advanced use-cases. We can consider separately which other things we would want to include in the WordPress version of the class.

Developers are free to choose either class, depending on their preferences. There are no differences in their feature set, the only difference is the exception vs WP_Error piece.