Run CI build with --enable-frozen-string-literal Ruby command line option

[floehopper]

Some useful context in https://gist.github.com/fxn/bf4eed2505c76f4fca03ab48c43adc72.

[floehopper]

The need to do this might go away when Ruby v3.4 is released, because then String literals will be frozen by default.

Also I have just fixed a bunch of relevant warnings in #672.

[floehopper]

The need to do this might go away when Ruby v3.4 is released, because then String literals will be frozen by default.

This doesn't appear to be true. Instead, any string literals which are not specifically marked as frozen will display a warning if mutated, but they can still be mutated.

This suggests it might be a good idea to add the magic comment to all files: # frozen_string_literal: true...?