Add UncaughtThrowError signatures

stdlib/builtin/errors.rbs

@@ -445,6 +445,8 @@ end
 # 
 #     UncaughtThrowError: uncaught throw "foo"
 class UncaughtThrowError < ArgumentError
+  def tag: () -> untyped
+  def value: () -> untyped
 end
 
 # Raised when attempting to divide an integer by 0.

test/stdlib/UncaughtThrowError_test.rb

@@ -0,0 +1,22 @@
+require_relative "test_helper"
+
+class UncaughtThrowErrorTest < StdlibTest
+  target UncaughtThrowError
+  using hook.refinement
+
+  def test_tag
+    begin
+      throw :a
+    rescue UncaughtThrowError => error
+      error.tag
+    end
+  end
+
+  def test_value
+    begin
+      throw :a
+    rescue UncaughtThrowError => error
+      error.value
+    end
+  end
+end