document configuration options and convert all of them to scoped values

[Krastanov]

Currently we have

1. f(...; mode=[:compressed|:expanded]) setting whether the layout of the gates is compressed to the left, such that independent gates are not drawn one after the other but rather in parallel. This should be converted to a scoped value and documented.
2. Quantikz.classicalbitslayout which is a scoped value that controls whether there should be one classical wire for all bits, or one wire per bit.
3. These scoped values should all be stored in a configuration structure (like explained here https://docs.julialang.org/en/v1/base/scopedvalues/#Very-many-ScopedValues ) that is marked public but not exported