Expose "reset" color value in themes

[wildwestrom]

Seems like this was a pretty trivial fix, but please let me know if you spot anything obviously wrong here.

I tested it and it seemed like you could pretty easily modify reset to whatever you want and have it show up in your theme.

[wildwestrom]

Since I can't put this in the title, this fixes #8075 to be clear.

[chtenb]

Perhaps the documentation could use a more extensive update.

The default palette uses the terminal's default 16 colors

That does no longer correspond to the list of identiefiers below it. I might send a PR at some point making some clarifications to this documentation chapter in general.

[chtenb]

I had a look at how the Reset value is used in crossterm, and I noticed that it's used for default colors, and does not correspond to the reset code as I assumed.

https://github.com/crossterm-rs/crossterm/blob/08762b3ef4519e7f834453bf91e3fe36f4c63fe7/src/style/types/colored.rs#L59

As you see, it's translated to code 39 and 49 which correspond to the default fg and bg colors.

In this light, shouldn't we rather expose this as "default" instead of "reset"? The phrase "reset" could then be reserved for the actual reset code (0), which resets all modifiers as well as the colors (bg and fg) to their defaults.