Add the gemspec for error_highlight

lib/error_highlight/error_highlight.gemspec

@@ -0,0 +1,27 @@
+# coding: utf-8
+lib = File.expand_path('../lib', __FILE__)
+$LOAD_PATH.unshift(lib) unless $LOAD_PATH.include?(lib)
+begin
+  require_relative "lib/error_highlight/version"
+rescue LoadError # Fallback to load version file in ruby core repository
+  require_relative "version"
+end
+
+Gem::Specification.new do |spec|
+  spec.name          = "error_highlight"
+  spec.version       = ErrorHighlight::VERSION
+  spec.authors       = ["Yusuke Endoh"]
+  spec.email         = ["mame@ruby-lang.org"]
+
+  spec.summary       = 'Shows a one-line code snippet with an underline in the error backtrace'
+  spec.description   = 'The gem enhances Exception#message by adding a short explanation where the exception is raised'
+  spec.homepage      = "https://github.com/ruby/error_highlight"
+
+  spec.license       = "MIT"
+  spec.required_ruby_version = Gem::Requirement.new(">= 3.1.0")
+
+  spec.files = Dir.chdir(File.expand_path(__dir__)) do
+    `git ls-files -z`.split("\x0").reject { |f| f.match(%r{\A(?:test|spec|features)/}) }
+  end
+  spec.require_paths = ["lib"]
+end