Improve the type for os.date using overloads

[JohnnyMorganz]

Improves the type of os.date based on its documentation https://create.roblox.com/docs/reference/engine/libraries/os#date

We know that the DateTypeResult table is only returned when the format string is either "*t" or "!*t", and otherwise its just a formatted string. This changes the type to showcase this appropriately.

I put the more strict type first in the overload because the ordering makes a difference

Note: the API docs used in studio (https://github.com/MaximumADHD/Roblox-Client-Tracker/blob/roblox/api-docs/en-us.json) may need to be updated to reflect the overloaded type instead.

It may be possible to add a lint of this too to provide analysis similar to how string.format works with a FormatString error. Will leave that for a different PR though