Luau type exclusion and type selections

[Wunder_Wulfe]

Type exclusion and type selections allow developers to have more distinct control over the types which are or are not allowed in a complex environment.

What benefits this provides:

1. Specify what types a function or type template accepts, giving much cleaner and direct typing for complex functions with multiple parameter types.

2. Prevent certain values from being used in generic functions, types, or variables.

type VectorCl = Vector2 | Vector3
type AcceptedTypes = number | CFrame | VectorCl | Color3 | UDim2
local function Lerp<T:AcceptedTypes>(a: T, b: T, c: number): T
    return
        if typeof(a) == "number" then
        a + (b - a) * c
        else  -- implied that 'T' is one of the other types, aka `AcceptedTypes & (not number)`
        a:Lerp(b, c)
end

type SpecialAny = not nil
-- `any & (not nil)` is the same type in this case

local alwaysHasValue: SpecialAny = 2.0
if alwaysHasValue == nil then
-- alwaysHasValue: never
else
-- alwaysHasValue: SpecialAny (not nil)
end
alwaysHasValue = nil -- Type 'not nil' could not be converted into 'nil'

-- Assume Object and ObjectExt are two objects created of the same class, but ObjectExt  is a version of the object that does not support this operation
function Class.SpecialOperation<T:(Object & not ObjectExt)>(self: T)
    -- ...
end