Syntax sugar to make string literal function names clickable in an IDE

TLDR

Introduce syntactical sugar that makes string literals clickable in an IDE, if they refer to callables.

Background / history

In PHP and especially in Drupal, we often have string literals that correspond to actual functions:
E.g. a form builder function, a page callback in hook_menu(), etc.

A typical IDE (e.g. PhpStorm) can make such string literals clickable, and include them in "find usages", if they are in some way annotated as callable.

Problem

In typical Drupal code, a lot of string literals that do refer to callables are not clickable.

Solution / Proposed change

Introduce syntactic sugar to make them clickable.

Renderkit is a big enough module to justify having its own class for this.

Proposed API

Introduce a class with static methods, where the method parameters are annotated as 'callable'.
Class name and method name must be as short as possible, to minimize distraction.

This syntax should only be used in places where a minor indirection overhead does not matter.

use Drupal\renderkit\R;

function mymodule_menu() {
  // Clickable wildcard loader '%node'.
  $items['node/' . R::w('node_load') . '/mymodule'] = [
    // Clickable page callback.
    'page callback' => R::f('mymodule_node_page'),
  ];
}

Risks / Downsides

We might decide some day that this was a bad idea and awkward.

In this case we shall stop using the class, and perhaps mark it as deprecated.
This can be done with no disruption whatsoever.

The only cost is we are polluting an IDE's autocomplete list with a single-letter class.
(technically this is fine, the class is in a namespace)