Add frozen_string_literal: true comments and deprecate default_locale config accessor in favor of locale

[ColinDKelley]

This PR

• Adds frozen_string_literal: true magic comment to all Ruby files.
• Removes the .freeze from string literals since that now happens automatically with the above comment.
• Fixes a bug where the :pt locale was pluralizing the shared "'lhão'" string literal (with gsub!) inside the LOTS constant.
• Deprecates default_locale config accessor in favor of simply locale. (For compatibility the old accessors are still supported by alias.)
• Updates README for the above.

Motivation for the deprecation: It was inconsistent that the locale config had a default_ prefix, but decimals_as didn't. The README referred the them both as defaults, which makes sense...that's how configs usually work. So I figured the easy way to make them consistent was to drop the default_ prefix. Alternatively, we could add a default_ prefix to decimals_as. Or if it seems controversial, we could just drop the commit from this PR and consider it separately.

[radar]

Let's remove that default_locale change from this PR, as I don't want to break anyone's code that's using this option.

[ColinDKelley]

Let's remove that default_locale change from this PR, as I don't want to break anyone's code that's using this option.

That commit doesn't break any code, because it has aliases for the previous names default_locale and default_locale=, along with specs. Still, I can remove it if you don't want it.

[radar]

Oops, sorry that I missed that alias! Ok. I think it's fine to keep in.