New rule: `Shared else-value for if/case`

Rule summary

From:

      (if (symbol? level)
          (case level
            [(warning)                                'warning]
            [(danger)                                 'danger]
            [(info success warning danger
                   primary secondary light dark)      level]
            [else                                     'info])
          'info)

To:

          (case level
            [(warning)                                'warning]
            [(danger)                                 'danger]
            [(info success warning danger
                   primary secondary light dark)      level]
            [else                                     'info])

The observation is that the value return by the else-arm of the if-expression,
is the same as the expression returned in the else-expression of the case.

Test case

#lang resyntax/test

test: "original code should be refactorable to new code"
--------------------
#lang racket
(define level 'foo)
(if (symbol? level)
    (case level
      [(warning) 'warning]
      [else      'info])
    'info)

====================
#lang racket
(define level 'foo)
(case level
      [(warning) 'warning]
      [else      'info])
--------------------

No-change test case

Additional context

Spotted here:

https://github.com/soegaard/webracket/blob/main/lib/web-easy/renderer.rkt#L892

Start by locating the existing refactoring-rule implementations and their test suite; the issue provides a positive Racket test case and points to renderer.rkt in the webracket repository as the observation site. Done means the shown if/case form is transformed into the case form while unrelated code remains unchanged.