(PUP-11326) Make regsubst() sensitive-aware

[cocker-cc]

PUP-11326

Possible Use-Case out of Real-Life:

      $_ssl_key = $::facts['os']['family'] ? {                           
        'windows' => regsubst($ssl_key, '\n', "\r\n", 'EMG'),            
        default   => $ssl_key,                                           
      }

[puppetlabs-jenkins]

Can one of the admins verify this patch?

[CLAassistant]

All committers have signed the CLA.

[joshcooper]

Thanks @cocker-cc you'll want to rebase this PR against #8879 after it's merged to resolve the rspec issue on Ruby 3

[hlindberg]

There is one more possible Sensitive permutation to support - that of Sensitive[Array] where the array may contain sensitives or strings. That case may be difficult to describe in the dispatch section since a Sensitive type does not present more than the base type of its content. Easiest is probably to have one dispatch for Sensitive[Array] that unwraps the Array and then recursively calls the function to get the type checking.
Not sure how valuable it is to also support that case.

[mhashizume]

Thanks for your contribution @cocker-cc !