This patch fixes the `i18n:check` rake task which has been broken by
the `MessageFormat` upgrade.
It also adds a spec to ensure we generate valid MF code for all our
available locales.
* File.exists? is deprecated and removed in Ruby 3.2 in favor of
File.exist?
* Dir.exists? is deprecated and removed in Ruby 3.2 in favor of
Dir.exist?
This reduces chances of errors where consumers of strings mutate inputs
and reduces memory usage of the app.
Test suite passes now, but there may be some stuff left, so we will run
a few sites on a branch prior to merging
