[Feature #21109]
By always freezing when setting the global rb_rs variable, we can ensure
it is not modified and can be accessed from a ractor.
We're also making sure it's an instance of String and does not have any
instance variables.
Of course, if $/ is changed at runtime, it may cause surprising behavior
but doing so is deprecated already anyway.
Co-authored-by: Jean Boussier <jean.boussier@gmail.com>
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