[External Cursor API] alphaTex Support for Sync Points

[Danielku15]

alphaTex is lacking of a way to specify sync points. We need one for easy syncing of songs written in alphaTex.

Consideration: Its unlikely that people will write the sync points manually. A compact syntax compatible with a FlatSyncPoint[] and applyFlatSyncPoints should be enough. Then we can generate the alphaTex code in the web editor. Manual writing is still possible if somebody wants.