Add well-formed timetable validation check

[kokrui]

Right now, NUSMods just checks on load for whether a timetable object exists. We could implement a basic check that the data it contains is well-formed in some way

https://github.com/nusmodifications/nusmods/blob/089993f56557c729572284b762384424f7806d53/export/src/data.ts#L66-L71