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

Nobody has claimed this yet.

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.

```lua
---@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!

Start with the Foo/Bar Lua example in the issue and trace how @return annotations and inherited methods are interpreted. Done means a constructor inherited by Bar infers its result as Bar rather than Foo, with coverage for the demonstrated behavior.

Written by the indexing model from the issue text.