[Investigation] Add JEP check to the PR

[mark.reinhold at oracle.com]

I have to ask: Is this really worth the trouble?

We’ve targeted and integrated over three hundred JEPs since the
inception of the process in 2011.  How many of those, besides JEP
295, were integrated prematurely?

The solution you offer requires the JEP implementor to remember to
type the `/jep` command in the PR.  They could forget to do that
just as easily as they forget to check the targeting status of the
JEP itself.

Now if you could automagically determine that a PR issue is a JEP
implementation and block it when the JEP is not targeted, without
the JEP implementor having to do anything special, then that might
be more compelling -- but still, is it a problem worth solving?

- Mark