It turns out that auto-highlight-symbol is a bit more advanced, with a MUCH nicer edit symbol feature than highlight-symbol does.
Also provide `M-x siren-doom-themes-load` to interactively choose a doom theme to load and apply my custom set of tweaks.
I've been using doom-themes' doom-vibrant for long enough now that I'm certain I won't be moving away from it anytime soon. So let's simplify and strip away all other theme setup stuff.
