Add signature for RubyVM::InstructionSequence.of

https://docs.ruby-lang.org/en/4.0/RubyVM/InstructionSequence.html#method-c-of

`prism` uses it and currently vendors this change.

The return type is nillable as per the implementation in ruby but I'm not sure how to actually trigger that for valid input.
It may happen with some procs, that's all I know.

Author: Earlopain

core/ruby_vm.rbs

@@ -371,6 +371,46 @@ class RubyVM::InstructionSequence < Object
   # event_symbol] pair.
   #
   def trace_points: () -> Array[untyped]
+
+  # <!--
+  #   rdoc-file=iseq.c
+  #   - of(p1)
+  # -->
+  # Returns the instruction sequence containing the given proc or method.
+  #
+  # For example, using irb:
+  #
+  #     # a proc
+  #     > p = proc { num = 1 + 2 }
+  #     > RubyVM::InstructionSequence.of(p)
+  #     > #=> <RubyVM::InstructionSequence:block in irb_binding@(irb)>
+  #
+  #     # for a method
+  #     > def foo(bar); puts bar; end
+  #     > RubyVM::InstructionSequence.of(method(:foo))
+  #     > #=> <RubyVM::InstructionSequence:foo@(irb)>
+  #
+  # Using ::compile_file:
+  #
+  #     # /tmp/iseq_of.rb
+  #     def hello
+  #       puts "hello, world"
+  #     end
+  #
+  #     $a_global_proc = proc { str = 'a' + 'b' }
+  #
+  #     # in irb
+  #     > require '/tmp/iseq_of.rb'
+  #
+  #     # first the method hello
+  #     > RubyVM::InstructionSequence.of(method(:hello))
+  #     > #=> #<RubyVM::InstructionSequence:0x007fb73d7cb1d0>
+  #
+  #     # then the global proc
+  #     > RubyVM::InstructionSequence.of($a_global_proc)
+  #     > #=> #<RubyVM::InstructionSequence:0x007fb73d7caf78>
+  #
+  def self.of: (Proc | Method | UnboundMethod body) -> RubyVM::InstructionSequence?
 end
 
 # <!-- rdoc-file=ast.rb -->

test/stdlib/RubyVM_InstructionSequence_test.rb

@@ -0,0 +1,16 @@
+require_relative "test_helper"
+
+class RubyVM::InstructionSequenceSingletonTest < Test::Unit::TestCase
+  include TestHelper
+
+  testing "singleton(::RubyVM::InstructionSequence)"
+
+  def test_of
+    assert_send_type "(::Method body) -> ::RubyVM::InstructionSequence",
+                     RubyVM::InstructionSequence, :of, method(:test_of)
+    assert_send_type "(::UnboundMethod body) -> ::RubyVM::InstructionSequence",
+                     RubyVM::InstructionSequence, :of, self.class.instance_method(:test_of)
+    assert_send_type "(::Proc body) -> ::RubyVM::InstructionSequence",
+                     RubyVM::InstructionSequence, :of, -> { }
+  end
+end