This change adds a field to the schema of themes which takes a
list of styles.

    rainbow = ["red", "orange", "yellow", { modifiers = ["reversed"] }]
    [palette]
    red = "#ff0000"
    orange = "#ffa500"
    yellow = "#fff000"

Normal style rules apply for each element in `rainbows`: you can
use definitions from the palette and the full fg/bg/modifiers
notation.

Themes written with `rainbow` keys are not backwards compatible.
Parsing errors will be generated for older versions of Helix
attempting to use themes with `rainbow` keys.

A default rainbow is provided with base16 colors.

This change is made with rainbow pair characters (parens, brackets, etc.)
in mind but it could also be used for other rainbow cosmetic elements
like rainbow indent-guides.

Docs

fmt