Support type inference via IsA

[shirovian]

foo:IsA("Part") proves that foo is a Part but this code produces this error instead:

local foo = workspace:FindFirstChild("Foo")

if foo and foo:IsA("Part") then
    print(foo.Position) -- Roblox LSP Type Checking: "Key `Position` not found in type `Instance`."
end

Here it could be inferred that foo IsA Part, this would be an amazing quality of life change, I constantly have to write boilerplate to convince the type checker it will be of the already IsA asserted class in these scenarios.

[Stefanuk12]

Additionally, FindFirstChild should return an optional type, and checking that it's not nil should resolve the proper Instance type.

[DazorPlasma]

Would really like to see this fixed. Currently it's a dealbreaker.