Improve developer experience

[olivier-thatch]

This PR sets up the repo to provide an improved developer experience:

1. Include a basic .editorconfig, because EditorConfig is awesome.

2. .ruby-version and Gemfile.lock are now committed

   • In the past, common practice was to not include these files for gems/libraries, but the current best practice is to commit these to provide a unified developer experience (everyone is running the same set of dependencies) and help with reproducibility.
   • No Ruby version manager is mandated. .ruby-version can be read by rbenv, rvm, mise, etc.
   • .ruby-version is set to 3.3.11, the latest in the 3.3 branch which is the oldest non-EOL branch. This helps prevent contributors from using modern Ruby features not available in the oldest supported version.
   • Gemfile now includes ruby file: '.ruby-version' to keep everything in sync
   • Gemfile.lock is generated with --normalize-platforms and --add-checksums to ensure it works on all platforms and that dependencies are pinned with their checksums (pretty important in this age of constant supply chain attacks)

3. VSCode (and VSCode-compatible editors like Cursor etc.) settings:

   • recommended extensions:
     • most importantly Shopify's excellent Ruby LSP
     • EditorConfig
     • GitLens
     • Open in GitHub
   • configure VSCode to format on save. The Ruby LSP will automatically apply Rubocop fixes based on the project's Rubocop configure

4. Ran rubocop --auto-gen-config to keep .rubocop_todo.yml up-to-date.

[dblock]

I don't kind things like EditorConfig or .vscode, will leave open for a bit for others to chime in. Generally in the copilot/claude world we care less and less. I do recommend splitting these into separate PRs so we can have separate CHANGELOG entries.

[dblock]

This is a gem, so we don't commit a lock file.