Add a way of marking a function as overidable

Some engines/libraries let user override global functions, but then adding API headers for them gives a "duplicate field" warning.

eg

Header file for engine API:

--- Engine API
global_table = {}

---Overidable function
---@param test_data string
---@return boolean
function global_table.do_stuff(test_data) end

Code that use this engine:

--- Currently raises the duplicated field warning
function global_table.do_stuff(test_data)
    -- Handle stuff differently
end 

The issue names no files, tests, or entry points. Start by tracing how duplicate-field warnings are produced for Lua functions and review existing annotation or override handling. Done means a documented way to mark an API function as overridable so user implementations no longer trigger the duplicate-field warning.