Add source_code_uri (+ other metadata) to gemspec (#239)

This is necessary in order to get this gem to show up on tea.xyz.

Source:
<https://docs.tea.xyz/tea/i-want-to.../begin-earning-tea-and-interact-with-the-protocol/registering-an-oss-project#updating-your-url-based-on-your-package-manager>

super_diff.gemspec

@@ -13,6 +13,12 @@ Gem::Specification.new do |s|
     SuperDiff is a gem that hooks into RSpec to intelligently display the
     differences between two data structures of any type.
   DESC
+  s.metadata = {
+    "bug_tracker_uri" => "https://github.com/mcmire/super_diff/issues",
+    "changelog_uri" =>
+      "https://github.com/mcmire/super_diff/blob/main/CHANGELOG.md",
+    "source_code_uri" => "https://github.com/mcmire/super_diff"
+  }
   s.required_ruby_version = ">= 3"
 
   s.files = %w[README.md super_diff.gemspec] + Dir["lib/**/*"]