Allow @generic function to return new type inherited from T

[paul-reilly]

I don't know if this is even possible, but it's always worth asking to see if it is!

So currently, we can have:

---@generic T
---@param type T
---@return T
function createNewType(type) end

-- this allows this *syntax* to work perfectly (which is important by itself)...
Foo = createNewType(Bar)
-- ... but Foo is just type Bar unless we annotate...

---@class Foo : Bar
Foo = createNewType(Bar)
-- Foo is now type Foo that inherits from Bar.

Now, if we could somehow enhance the @generic annotation of the type creation function we could remove the need for that last annotation:

---@generic T
---@param type T
---@return :T   --<-- the inheritance colon here (for example) would be a directive that the assignee will be a type that inherits from T
function createNewType(type) end

Foo = createNewType(Bar) 
-- Foo is now type Foo that inherits from Bar, without annotation at the site of type creation

This and #1505 would be good for various OOPy patterns.