Move executables to exe folder (#1366)

Conventionally `bin` folder is home to scripts that are meant to be run by developers working on the project, whereas the `exe` folder is used for executables that are meant to be exported by the gem.

The guidance from the Bundler team around this has been live since 2015: https://bundler.io/blog/2015/03/20/moving-bins-to-exe.html

This PR simplifies the gemspec and updates the `bin` folder to `exe` in the gemspec and removes some of the unnecessarily cleanup that was introduced previously.

stripe.gemspec

@@ -33,13 +33,12 @@ Gem::Specification.new do |s|
     /\A\.rubocop/,
     /\A\.travis.yml/,
     /\A\.vscode/,
-    %r{\A(bin/tapioca)},
+    /\Abin/,
     /\Asorbet/,
     /\Atest/
   )
-  s.files = `git ls-files`.split("\n").reject { |f| ignored.match(f) }
-  s.executables   = `git ls-files -- bin/*`.split("\n")
-                                           .map { |f| File.basename(f) }
-                                           .reject { |f| f == "tapioca" }
+  s.files = `git ls-files`.split("\n").grep_v(ignored)
+  s.bindir = "exe"
+  s.executables   = `git ls-files -- exe/*`.split("\n").map { |f| File.basename(f) }
   s.require_paths = ["lib"]
 end