Formalize deephaven express color_continuous_scale values allowed

[jnumainville]

color_continuous_scale can take str | list[str]

The strings it takes are from a limited Literal set, and we should specify which literals can be passed in, such as "plasma" or "viridis".

Additionally, px exposes colors directly, such as in the case of px.colors.sequential.Viridis
We should do that as well, something like dx.colors.sequential

Some context here