score column name is confusing

[changhiskhan]

Conventionally, score means "similarity", which means higher the better.
But Lance returns score as the distance metric function, which means lower the better.
This has resulted in multiple instances of user confusion.

[eddyxu]

When we decide to change the name, let's also change to name a "meta column," i.e., staring with a underscore "_distance" or something, indicating that this column does not physically exist.