Add 'prepend' support to RSpec configuration

RE: #1789 
There was some remaining ambiguity as to what the best approach was for this feature on non-compatible versions. I went the route of checking for a `prepend` definition on `Module` and otherwise raising a `NotImplementedError`; if explicitly checking the `RUBY_VERSION` is the preferred route let me know. For the specs, I conditionally executed them based on the same criteria.
We have [specific code](https://github.com/rspec/rspec-support/blob/7692f2748e8da5d1e05d71674193689e75ea8fd6/lib/rspec/support/ruby_features.rb#L107) in rspec-support for this check.

We also prefer to do this as a conditional method definition... e.g.

```Ruby
if RSpec::Support::RubyFeatures.module_prepends_supported?
  def safe_prepend(mod, host)
    ...
  end
else
  def safe_prepend(_, _)
  end
end
```

as it avoids an at runtime check, personally I'd define this to be a no op on other Rubies. I'd also like to define the config method the same way, and raise the error there, so it's raised when config is attempted rather than at runtime.