Feature request: `pal_between()`

This FR is for a simple discrete palette that does the following:

```r
pal_between <- function(min, max) {
  force(min)
  force(max)
  new_discrete_palette(
    function(n) seq(min, max, length.out = n),
    type = "numeric"
  )
}
```

It'd be convenient for some discretisation of continuous values with a known range.