Ensure compatibility with `--enable-frozen-string-literal`

[casperisfine]

Ref: https://bugs.ruby-lang.org/issues/20205

In Ruby 3.4 string literals will be "chilled" by default. Meaning they are still mutable, but will pretend to be frozen, and emit a warning when mutated.

Then a later version of Ruby will default to frozen string literals.

NB: I'd happily add CI coverage but https://github.com/abachman/ulid-ruby/pull/7 would be needed first.