Theme preview doesn't show "string" label when a closure is used in `color_config`

Trying out an example from @fdncred where:

$env.config.color_config.string = {|x| if $x =~ '^#[a-fA-F\d]+' { $x } else { 'default' } 

If that's set in a theme, the theme preview small shows the closure, but doesn't label it as the string type. All the types with basic (or record) colors show the labels.

On the bright side, none of the current themes use a closure for anything other than date, filesize, and bool, and all of those expand their values instead of showing the closure value. Still, if someone has their color_config set like this, the theme preview will be off. Even if the label was shown properly, attempting to display the closure itself inside the table is going to be problematic from a formatting standpoint. Probably need a different solution here.

Reproduce the issue by setting a closure in color_config.string and running theme preview small. Read the theme preview handling for closure-valued colors; done means the string label is represented correctly without trying to format the closure itself.