Allows a higher-level prefixes and suffixes by allowing it directly on
`@theme-*` options, which are what are set directly against Tmux's
options.
These are the prefix/suffix options added:
- `@theme-status-left-prefix`
- `@theme-status-left-suffix`
- `@theme-status-right-prefix`
- `@theme-status-right-suffix`
- `@theme-window-status-current-prefix`
- `@theme-window-status-current-suffix`
- `@theme-window-status-prefix`
- `@theme-window-status-suffix`
As they are applied on the highest level, it means they work on ALL
themes, including the `default` theme.
The `@themepack-*` prefix/suffix options still work too, but only for
themes that uses them and have three separate text areas on the left and
right status.
Use Tmux itself by spinning up a temporary test server, load the theme,
and use "show-options" to pull out and validate relevant options that
are set by the theme.
