(#19233) No variable interpolation for regex

Currently, the Datatype docs claim that:

> Regular expressions are written as standard Ruby regular expressions (valid
> for the version of Ruby being used by Puppet)

However a notable feature of forward-slash quoted Ruby regular expressions that
we do not support is variable interpolation.
Merge pull request #145 from Sharpie/19233-no-variable-interpolation-for-regex

(#19233) No variable interpolation for regex