Rename to Stevia

p-o-s is more than a stub since some time. To reflect that in the name we rename to Stevia (as it makes typing sweet). This allows us to keep the symbol prefix at pos_. This MR does the rename but keeps repo name and URLs unchanged thus after migration we'll

• rename the repo
• do a follow up MR to adjust for the repo rename (at this point things will remain to work as gitlab redirects from the old repo name)

I intend to merge this past the 0.47 release so we have the whole 0.48 to address any possible fallout.

Would be cool to also have a nice icon at some point if someone feels like it.

Thanks to @Newbyte for suggesting the name.