PlaceholderAPI Implementation

[FlashyReese]

Originally brought up in #46.

Targeted branch: https://github.com/FlashyReese/CommandAliases/tree/placeholders-api-impl

• Replace in-house function processor with Placeholder API to reduce maintenance and bug-fixing efforts. Keep the deprecated function processor for a few releases to allow users to adjust to the change.
• Transition all functions to use Placeholder API
  • Implement dynamic passing of functions to Placeholder API for servers
  • Implement dynamic passing of functions to Placeholder API for clients
    • Wait for Placeholder API to support FabricClientCommandSource as a context
• Update documentation on the wiki to reflect the changes made

[FlashyReese]

This will only be implemented for 1.19.3 and above. Unless someone is willing to backport and maintain older branches.

[tanyan-tanyan]

How come you're not implementing this for 1.19.2? (Pls be gentle I don't know much about this stuff)