Add a way of marking a function as overidable

[AFCMS]

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 

[BribeFromTheHive]

Can confirm. Looks like this can be merged with the request here: #1794

[Papipo]

Is there a workaround for this?

I just have something like:

local mt = {}
mt.__index = mt

funcion mt.foo() 
  -- default implementation here
end

local extension = setmetatable({}, mt)
function extension.foo() -- raises duplicate-set-field

Which AFAIK is idiomatic lua.