#2742 extend is_function() with support for public local functions

[Anonymous]

Originally created by: Helianthella

Pull Request Prelude

• [X] I have followed proper Hercules code styling.
• [X] I have read and understood the contribution guidelines before making this PR.
• [X] I am aware that this PR may be closed if the above-mentioned criteria are not fulfilled.

Changes Proposed

This is a follow-up to [#2142]

• Adds FUNCTION_IS_PUBLIC to check if a local function is public
• Adds support for searching in other NPCs
• Adds unit tests