* Move `runtime/themes/base16_default_terminal.toml` to
`base16_theme.toml` alongside `theme.toml`
* Use `terminfo` crate to detect whether the terminal supports true
color and, if the user has no theme configured and their terminal does
not support true color, load the alt default theme instead of the
normal default.
Remove `terminfo` dependency, use `COLORTERM` env instead
Prevent user from switching to an unsupported theme
Add `true-color-override` option
If the terminal is wrongly detected to not support true color,
`true-color-override = true` will override the detection.
Rename `true-color-override` to `true-color`
