Add "identity" scales

"Identity" scale is a scale that takes aesthetics values directly and does not change them
For example:
```
color(listOf(Color.RED, Color.GREEN, Color.BLUE)) {
scale = identity()
}
```

The issue does not name implementation files or tests. Start by locating the existing scale definitions and their test coverage, then compare how they accept aesthetic values; done means an identity scale can be selected as shown and preserves the supplied Color values.

Written by the indexing model from the issue text.