Add support for `setopt`

[shackra]

hello

since or before Emacs 29.1, there is a function called setopt:

setopt is an autoloaded Lisp macro in cus-edit.el .

(setopt [VARIABLE VALUE]...)

Set VARIABLE/VALUE pairs, and return the final VALUE.
This is like setq , but is meant for user options instead of
plain variables. This means that setopt will execute any
custom-set form associated with VARIABLE.

Note that setopt will emit a warning if the type of a VALUE
does not match the type of the corresponding VARIABLE as
declared by defcustom . (VARIABLE will be assigned the value
even if it doesn t match the type.)

Since it is like setq, I thought that probably leaf.el should support it, and use setq for older versions of Emacs under the hood.

[conao3]

My understanding is that setopt uses the syntax of setq for custom-set-variables.
Since existing Leaf users already have a sufficiently complete configuration DSL through :custom, there's little strong motivation to support :setopt. PRs are welcome, of course.