Add parallel option

[aergonaut]

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

This PR adds a parallel option to I18nJs.call, which is passed to the export_group method. When parallel: true and the group uses :locale placeholder, export_group will use concurrent-ruby's Promise to parallelize exporting each file.

I have included a benchmark script in this PR which compares the time needed to export 100 locale files with 5_000 strings each. Here are some example results:

❯ ruby test/benchmarks/parallel_export.rb
              user     system      total        real
baseline  4.199457   0.144715   4.344172 (  4.376356)
parallel  1.999492   0.071311   2.070803 (  2.064344)

Parallelization makes the process takes about 50% less time.

Why

In an app with many strings and many locales, exporting can take a significant amount of time, because each locale is exported sequentially. This slows down the feedback loop, making developers wait longer to see the effects of a change.

Known limitations

[TODO or N/A]