Classed palettes with properties

[teunbrand]

This PR aims to fix #426.

Briefly, it introduces two classes of palettes: <pal_continuous> and <pal_discrete>, which I mostly see as making {scales}' palettes more formal.

The classes are simply functions with attributes that provide information about the palette.
The <pal_discrete> class accepts a scalar n as input and spits out a vector of n distinct values.
The <pal_continuous> class accepts a numeric vector between (0, 1) and spits out a vector of equal length.
All existing palettes have been given appropriate classes.

The palette_type() function probes the type attribute, indicating what palettes can be used for, like "colour" or "linetype".
The palette_na_safe() function probes <pal_continuous> on whether NA are translated (e.g. in using the colour_ramp(na.colour) argument).
The palette_nlevels() function probes <pal_discrete> what the maximum n is.

The as_continuous_pal() function can cast <pal_discrete> to <pal_continuous>. For colour types, this is wrapping the discrete palette in colour_ramp().
The as_discrete_pal() function can cast <pal_continuous> to <pal_discrete>. This is mostly palette_fun(seq(0, 1, length.out = n).

A small demo:

devtools::load_all("~/packages/scales")
#> ℹ Loading scales

# Probing properties
pal <- pal_brewer("qual")
is_discrete_pal(pal)
#> [1] TRUE
palette_nlevels(pal)
#> [1] 8

# Converting from continuous to discrete
pal <- pal_gradient_n(rainbow(10))
is_continuous_pal(pal)
#> [1] TRUE
disc <- as_discrete_pal(pal)
is_discrete_pal(disc)
#> [1] TRUE
show_col(disc(9))

^{Created on 2024-04-16 with reprex v2.1.0}

[teunbrand]

@thomasp85 can I put this on your radar for review sometime?

[thomasp85]

LGTM