Add weekday parsing and two-letter weekday shorthands

[pcekm]

@LennartCode asks in #3:

On that note, it would also be nice to add dedicated methods for parsing Weekdays and a getter for accessing two-letter weekday shorthands. Those are minor quality of life features and unrelated to the actual proposal. I would be happy to contribute them if you deem them in the scope of the package though!

[pcekm]

I don't mind adding weekday name parsing, but I'm not so sure about two-letter weekday shorthands. They sound locale or application specific. I'd much prefer a parse function that used the name field.

[LennartCode]

Yes, weekday shorthands are actually locale specific. I forgot about that and they are definitely out of scope. I will provide an initial parsing implementation for further discussion.