Support passing a category to Warning.warn

This change adds a `category` kwarg to make it easier to monkey patch
`Warning.warn`. Warnings already have a category, but that warning isn't
exposed. This implements a way to get the category so that warnings with
a specific category, like deprecated, can be treated differently than
other warnings in an application.

The change here does an arity check on the method to support backwards
compatibility for applications that may already have a warning monkey
patch.

For our usecase we want to `raise` for deprecation warnings in order to
get the behavior for the next Ruby version. For example, now that we
fixed all our warnings and deployed Ruby 2.7 to production, we want to
be able to have deprecation warnings behave like they would in 3.0: raise
an error. For other warnings, like uninialized constants, that behavior
won't be removed from Ruby in the next version, so we don't need to
raise errors.

Co-authored-by: Aaron Patterson <tenderlove@ruby-lang.org>

test/ruby/test_exception.rb

@@ -915,7 +915,7 @@ $stderr = $stdout; raise "\x82\xa0"') do |outs, errs, status|
     end
   end
 
-  def capture_warning_warn
+  def capture_warning_warn(category: false)
     verbose = $VERBOSE
     deprecated = Warning[:deprecated]
     warning = []
@@ -924,8 +924,14 @@ $stderr = $stdout; raise "\x82\xa0"') do |outs, errs, status|
       alias_method :warn2, :warn
       remove_method :warn
 
-      define_method(:warn) do |str|
-        warning << str
+      if category
+        define_method(:warn) do |str, **kw|
+          warning << [str, kw[:category]]
+        end
+      else
+        define_method(:warn) do |str|
+          warning << str
+        end
       end
     end
 
@@ -954,6 +960,18 @@ $stderr = $stdout; raise "\x82\xa0"') do |outs, errs, status|
     assert_equal(["\n"],     capture_warning_warn {warn ""})
   end
 
+  def test_warn_backwards_compatibility
+    warning = capture_warning_warn { Object.new.tainted? }
+
+    assert_match(/deprecated/, warning[0])
+  end
+
+  def test_warn_category
+    warning = capture_warning_warn(category: true) { Object.new.tainted? }
+
+    assert_equal :deprecated, warning[0][1]
+  end
+
   def test_kernel_warn_uplevel
     warning = capture_warning_warn {warn("test warning", uplevel: 0)}
     assert_equal("#{__FILE__}:#{__LINE__-1}: warning: test warning\n", warning[0])