feat: ImGui lua bindings

[TechHog8984]

example:

local ImGui = game:GetService("ImGuiService")

local window_active = Instance.new("BoolValue")
window_active.Value = true

local render_connection
render_connection = ImGui.Render:Connect(function()
    if ImGui.Begin("Lua Window", window_active) then
        ImGui.Text(("Hello, %s"):format("world!"))
        if ImGui.Button("Destroy") then
            window_active.Value = false
        end
    end
    ImGui.End()

    -- disconnecting the connection should go after the button press because
    -- we don't want to omit the above End midway through
    if not window_active.Value then render_connection:Disconnect() end
end)

[TechHog8984]

The foundation has been laid for this (the above example works and there is more functionality than shown), however there is quite a bit missing (in terms of both which widgets exist as well as general things like flags or styling).