Initial sinatra project structure

Gemfile

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+source 'https://rubygems.org'
+
+gem 'sinatra', '~> 3.0'
+gem 'sinatra-contrib', '~> 3.0'
+gem 'puma', '~> 6.3'
+
+gem 'sequel', '~> 5.70'
+gem 'sqlite3', '~> 1.7'
+
+group :development, :test do
+  gem 'rerun'
+  gem 'wdm', '>= 0.1.0' if Gem.win_platform?
+
+  # rubocop and extensions for code style
+  gem 'rubocop'
+  gem 'rubocop-sequel'
+end