ruby : Fix of C++ header guard name, model URI support, type signature and more ...

ruby : Fix of C++ header guard name, model URI support, type signature and more (#2683)

* Add test to make Whisper::Context.new accept URI string

* Add test to make Whisper::Context.new accept URI

* Make Whisper::Context.new accept URI string and URI

* Update README

Revert "Fix argument of rb_undefine_finalizer"

* Fix typos

* Add type signature file

* Assign literarl to const variable

* Load Whisper::Model::URI from Init_whisper

* Simplify .gitignore

* Don't load whisper.so from whisper/model/uri.rb

* Use each_with_object instead of each

* Add Development section to README

* Rename header guard to conform to C++ naming convention