* [core] add z$ as Evil keybinding for set-selective-display
This is a feature whose equivalent I had looked for previously and not found. I
bumped into it in [this excellent
article](https://karthinks.com/software/batteries-included-with-emacs/) which is
a survey of "batteries included" parts of Emacs. He mentions Spacemacs (in a
positive light) in the article, but the article is a lot about what you can do
_without_ distributions like Spacemacs, Doom, or Prelude. As such, this seems
like an opportunity to make Spacemacs even better---i.e. by making these
relatively unknown batteries that are already present in Emacs more discoverable
in Spacemacs.
I put it in the folding menu `z` because that fits its usecase. I used `$`
because:
* the out-of-the-box Emacs keybindings is `C-x $`.
* the menu is already fairly saturated
* I didn't know what else to base the mnemonic on because
* I'm not aware of a Vim equivalent
* The name of the Emacs function itself is nondescriptive of its behavior
I couldn't find any keybinding documentation table to add it to. Please let me
know if I missed something.
* [core] DWIM in z$ as suggested by @lebensterben
af078784b8