Resolves #478

Just a simple proof of concept for how the ideas in #478 could look. This adds in a ThemeDefaults enum and a Theme::load_default() constructor that can handle loading a single default theme

Random thoughts:

• bincode doesn't support easily ignoring values, so all themes leading up to the one you want are still loaded
  • See Convenience API for skipping data? bincode-org/bincode#154
    • Closed as stale, not because it is resolved
• ThemeDefaults should be #[non_exhaustive] if you want to support adding more default themes without a semver breaking change
• ThemeDefaults should probably derive more traits, but I left it barebones for the proof-of-concept
• ThemeDefaults::as_theme_name() wasn't exposed publicly because of the potential confusion that the theme name should match the theme.name.unwrap() which is not the case for some of the defaults (this confused me at least)
• Luckily this wasn't a schema change for default.themedump since bincode serializes a struct with a single field mapped to a value as just the single value

Overall this means that you can write code like this to load a single default theme which is nice when you're only dealing with single themes

use syntect::highlighting::{Theme, ThemeDefaults};

let inspired_gh = Theme::load_default(ThemeDefaults::InspiredGitHub);