[ruby/strscan] Implement StringScanner for TruffleRuby in pure Ruby

(ruby/strscan#195)

* Fixes ruby/strscan#194.
* This is a fresh new implementation from scratch, contributed directly
to ruby/strscan, under BSD-2-Clause.
This was implemented using the strscan tests, specs from ruby/spec and
the documentation
(https://docs.ruby-lang.org/en/master/StringScanner.html).
* lib/strscan.rb now handles the loading for all implementations.
* Test on TruffleRuby 33 to ensure it keeps working on older TruffleRuby
releases,
which do not have the necessary Primitives used by this new
implementation.

---------

ruby/strscan@3908e0ddb0

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

Author: 2 people

ext/strscan/lib/strscan.rb

@@ -0,0 +1,20 @@
+# frozen_string_literal: true
+
+case RUBY_ENGINE
+when 'ruby'
+  require 'strscan.so'
+  require_relative 'strscan/strscan'
+when 'jruby'
+  require 'strscan.jar'
+  JRuby::Util.load_ext('org.jruby.ext.strscan.StringScannerLibrary')
+  require_relative 'strscan/strscan'
+when 'truffleruby'
+  if RUBY_ENGINE_VERSION.to_i >= 34
+    require 'strscan/truffleruby'
+  else
+    $LOAD_PATH.delete __dir__
+    require 'strscan'
+  end
+else
+  raise NotImplementedError, "Unknown Ruby: #{RUBY_ENGINE}"
+end