Support auto complete of callback arguments where callback is defined via a local variable

[tomlau10]

I think this is somewhat similar to #460, where the difference is that the callback is defined as a local variable.

Example

Using the sample code in the above issue to illustrate the situation:

---@class Class
---@field x number
---@field y number

---@param callback fun(value: Class)
function work(callback)
end

local function f(value)
    value.$  -- no autocompletion here
end
work(f)

It would be great if f can be inferred automatically (given that f is not already manually annotated of course).