Fix cursor jumping issues when typing in the field

[leb2]

Test with :

npm run playground

This PR fixes issues where if you type into the datetime text box, it would sometimes cause the date string to update

For example, if you were typing into this text box. and your cursor was at the end, and you delete the "M", it would automatically re-populate the M because just the "a" is a validate date, and it would get normalized back to "AM".

There are other issues where if you type into the middle of the text box, and the date changes from being an invalid date to a valid date, it would sometimes move your cursor to the very end.

To fix this, I made a new variable which keeps track of what the user types, and which will get refreshed if the date changes from the outside.

[yagmin]

Working well, nice improvement.

I noticed the 2nd example displays seconds for editing in the GUI but seconds aren't shown in the formatted string. I believe this is only caused to the formatting settings for that example.