Ruby 3.x upgrade and version bump to 2.0.0

[JPDuchesne]

Summary

• Breaking: Drop Ruby 2.x support, require Ruby >= 3.2 (semver major bump to 2.0.0)
• Update parser (>= 3.0) and unparser (>= 0.6) for Ruby 3.x compatibility
• Bump ast_transform dependency to ~> 2.0
• Merge consecutive :str nodes in dstr transformations to satisfy unparser 0.8.x strictness
• Add .compact to process_rspock to handle nil children from empty class bodies
• Replace pry-byebug with pry, coveralls with simplecov

CI/CD

• Replace Travis CI with GitHub Actions (ci.yml: tests on Ruby 3.2–3.3, release.yml: automated gem publish on tag push via Trusted Publishing)
• Add Codecov integration with SimpleCov JSON formatter
• Release workflow validates that git tag matches version.rb before publishing

Tests

• Add test for dstr transformation when last child is not a :str node (covers SPACE_STR_AST append branch)
• Update test expectations to match unparser 0.8.x output formatting

Docs

• Update README badges (Travis/Coveralls → GitHub Actions/Codecov)
• Document release process (GitHub UI and CLI flows)
• Fix homepage URL in gemspec

Dependencies

• Requires ast-transform 2.0.0 to be merged and released first

Test plan

• All 129 tests pass locally on Ruby 3.3 (95.41% coverage)
• CI passes on Ruby 3.2, 3.3 (blocked until ast-transform 2.0.0 is merged)

Setup required after merge

1. Codecov: Sign in at codecov.io with GitHub, add repo, add upload token as CODECOV_TOKEN secret
2. Trusted Publishing: Configure gem on rubygems.org as a trusted publisher