Feature: EmmyLua-like metamethod documenting

[Feez]

It would be awesome if we included some type of EmmyLua documentation extension to support metamethods (particularly the Mathematic Operators).

I'd like to be able to code something like this:

---@class MyClass
---@metamethod add fun:(self: MyClass, other: MyClass) : MyClass
---@metamethod subtract fun:(self: MyClass, other: MyClass) : MyClass
local x = {}

---@type MyClass
local a

---@type MyClass
local b

--- Ideally, `c` would be inferred to be MyClass off of the annotations
local c = a + b

[Frityet]

This would be really useful for __call aswell!

local tbl = setmetatable({}, {
	__call = function (s) return s end
})

print(tbl("Hi"))

[sumneko]

see #599