Enhance to support 'Set' object as an enum

[kwatch]

OptionParser supports Array and Hash object as an enum of option value.
But Set object is not supported.
For example:

require 'optparse'

parser = OptionParser.new
parser.on("--lang1=<lang>", ["en", "fr", "it"])   # option value will be checked by enum values
parser.on("--lang2=<lang>", Set.new(["en", "fr", "it"]))   # will not be checked when Set object

opts = {}
parser.parse(["--lang2", "ja"], into: opts)   # !!! not raise error !!!
parser.parse(["--lang1", "ja"], into: opts)   # raises error expectedly
p opts

Since Ruby 3.2, Set class is a built-in class.
I think that it is very natural for OptionParser to support Set object.

By the way, I can't find a test script to test OptionParser#make_switch() method.
Therefore this pull request doesn't contain any test case.

[kwatch]

By the way, I can't find a test script to test OptionParser#make_switch() method.
Therefore this pull request doesn't contain any test case.

Added new test script test/optparse/test_switch.rb.
If you find something to improve in it, let me know.