Add "Extraction Game Mode"

[phikal]

As suggested on reddit the game should implement not only matching the whole string, but also splitting out a part. I would suggest it to start about level 7, with only pre or post strings on one side.

Stylistically it could be like ante- string -post with ante- and -post in a lighter color.

This would have to also be explained either in the start screen or when it appears for the first time

[phikal]

First part of implementation finished with eab91c1