`site.standard.(palette|theme).basic` should include luma/contrast/bright/dark #3

The current standard lets you specify a palette, but does not indicate if it is a light or dark palette.

To do this one needs to calculate Luma:

        filter GetLuma {
            $colorString = $_
            # Convert the background color to a uint32
            $rgb = ($colorString -replace "#", "0x" -replace ';') -as [UInt32]
            # then make it into a percentage red, green, and blue.
            $r, $g, $b = ([float][byte](($rgb -band 0xff0000) -shr 16)/255),
                ([float][byte](($rgb -band 0x00ff00) -shr 8)/255),
                ([float][byte]($rgb -band 0x0000ff)/255)
        
            # Calculate the luma of the background color
            [Math]::Round(
                0.2126 * $R + 0.7152 * $G + 0.0722 * $B,
                6
            )
        }

Once one has computed the background color luma, you know if the palette is Bright or Dark.

I normally use a cutoff of: if ($luma -gt 0.4) { "bright" } else {"dark"}

While you're there, calculating Contrast is also pretty straightforward:

[Math]::Abs(($foreground | GetLuma) - ($background | GetLuma))

Why does this matter?

Quite a few reasons, but @media queries and personal preferences spring first to mind.

With at least a Luma value, it's pretty easy to determine if a palette is bright or dark, and this makes it much easier to pick between palettes.

Also, as I write out this issue, I'm realizing it's a good additional justification for why it should be palette, not theme - a theme might include a bright or dark palette, but a palette is either bright or dark.