Problem:
"playground" is new jargon that overlaps with existing concepts:
"dev" (`:help dev`) and "view" (also "scratch" `:help scratch-buffer`) .
Solution:
We should consistently use "dev" as the namespace for where "developer
tools" live. For purposes of a "throwaway sandbox object", we can use
the name "view".
- Rename `TSPlayground` => `TSView`
- Rename `playground.lua` => `dev.lua`
