SlevomatCodingStandard.Functions.StaticClosure: provide setting to toggle for arrow functions or traditional closures

[montchr]

SlevomatCodingStandard.Functions.StaticClosure does not provide a setting to
toggle this sniff for only arrow functions or traditional function closure
syntax.

While declaring traditional closures static might be a best practice, the PHP
RFC introducing the arrow function implementation implies that declaring
static arrow functions is a micro-optimization in the majority of cases:

For normal closures, [$this binding] can be prevented by prefixing them with
static. For the sake of completeness this is also supported for arrow
functions...

and:

Static closures are rarely used: They're mainly used to prevent $this
cycles, which make GC behavior less predictable. Most code need not concern
itself with this.

https://wiki.php.net/rfc/arrow_functions_v2#this_binding_and_static_arrow_functions

Personally, I would prefer to avoid the addition of static for arrow functions,
as it seems they are, by design, intended to provide a concise syntax without
static. Since I don't particularly care about static for traditional closures,
I am fine with just removing this sniff from my configuration.

[kukulich]

I'm open to accept PR with new option in this sniff :)