Add llhttp-ffi gem (#1030)

* Add llhttp-ffi gem

Minimal stubs for LLHttp::Parser, LLHttp::Delegate, and LLHttp::Error —
just the surface referenced by the http gem's bundled sig/http.rbs
(class Handler < LLHttp::Delegate, attr_reader parser: LLHttp::Parser)
plus the Parser methods http's Response::Parser calls.

Unblocks Steep users of the http gem, which otherwise fails signature
load with RBS::NoSuperclassFoundError on LLHttp::Delegate.

* Simplify test by removing the Delegate subclass

Use LLHttp::Delegate.new directly instead of defining a subclass in the
test. Drops the companion test.rbs since there's no custom Ruby class
left to declare. Steep already validates the subclass case when it
loads http.rbs (class Handler < LLHttp::Delegate), so the test doesn't
need to exercise it too.

Author: hakanensari

gems/llhttp-ffi/0.5/_test/test.rb

@@ -0,0 +1,8 @@
+require "llhttp"
+
+parser = LLHttp::Parser.new(LLHttp::Delegate.new, type: :response)
+parser.reset
+parser << "HTTP/1.1 200 OK\r\n\r\n"
+parser.status_code
+parser.http_major
+parser.http_minor

gems/llhttp-ffi/0.5/llhttp-ffi.rbs

@@ -0,0 +1,17 @@
+module LLHttp
+  class Error < StandardError
+  end
+
+  class Parser
+    def initialize: (untyped delegate, ?type: Symbol) -> void
+    def reset: () -> void
+    def <<: (String data) -> void
+    def status_code: () -> Integer
+    def http_major: () -> Integer
+    def http_minor: () -> Integer
+  end
+
+  class Delegate
+    def initialize: () -> void
+  end
+end