Export translations in parallel.

[fnando]

PR Checklist

PR Structure

• This PR has reasonably narrow scope (if not, break it down into smaller
  PRs).
• This PR avoids mixing refactoring changes with feature changes (split into
  two PRs otherwise).
• This PR's title starts is concise and descriptive.

Thoroughness

• This PR adds tests for the most critical parts of the new functionality or
  fixes.
• I've updated any docs, .md files, etc… affected by this change.

What

Export translations in parallel.

Why

Supersedes #733.

Known limitations

N/A

[Copilot]

Pull request overview

This PR updates the Ruby exporter to write per-locale translation files concurrently when the output path uses the :locale placeholder, improving export performance and superseding the approach proposed in #733.

Changes:

• Parallelize per-locale export in I18nJS.export_group using concurrent-ruby promises.
• Add concurrent-ruby as a runtime dependency and document the behavior change in the changelog.
• Update Node/TypeScript tooling metadata (TypeScript version bump, add @types/node, add package-lock.json) and add a benchmark fixture ignore rule.

Reviewed changes

Copilot reviewed 5 out of 6 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
lib/i18n-js.rb	Runs per-locale write_file calls concurrently via Concurrent::Promises when :locale is present.
i18n-js.gemspec	Adds concurrent-ruby runtime dependency and adds a new dev dependency.
CHANGELOG.md	Notes the behavioral change: parallel export when :locale placeholder is used.
package.json	Bumps TypeScript and adds @types/node to dev dependencies.
package-lock.json	Introduces npm lockfile pinning dependency tree for the TS tooling.
test/fixtures/bench/.gitignore	Ignores generated benchmark YAML fixture files.

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