FR: Allow 'self' return type to allow return type changing with inheritance

When annotating that one class has inherited from another, there is currently no way to annotate a function that returns it's current type that the current type should change when the class is inherited. Something along these lines, or something that accomplishes the same goal would be fantastic.

---@class Foo
Foo = {}

---@return self    --<-- 'self' refers to Foo so Foo.new() returns Foo
function Foo.new() end

---@class Bar : Foo
Bar = {}

local b = Bar.new() --<-- Bar.new() is inherited so the 'self' return type is now Bar
-- b should now be type Bar

Thanks!

Issue の Foo/Bar Lua 例から始めて、@return アノテーションと継承されたメソッドがどのように解釈されるかを追跡します。Bar が継承したコンストラクターの結果が Foo ではなく Bar として推論され、示されている動作のカバレッジがある状態を完了とします。