RFR 8153402: jshell tool: completion provider for /help

[Robert Field]

Bugs:

     8153402: jshell tool: completion provider for /help
     https://bugs.openjdk.java.net/browse/JDK-8153402

     8169818: jshell tool: completion provider for /vars /methods /types 
gives -history
     https://bugs.openjdk.java.net/browse/JDK-8169818

Webrev:

     http://cr.openjdk.java.net/~rfield/8153402v0.webrev/

Thanks,
Robert