RFR: 8296454: System.console() shouldn't return null in jshell [v3]

[Jan Lahoda]

> Implements `System.console()` for JShell, based on JDK-8295803. It is only supported for (supported) remote agents.
> 
> When a snippet calls a Console method in the remote agent, a request to perform the task is sent to the main process, performed, and the outcomes are sent back to the remote agent, and returned from the Console method.
> 
> Please also review the CSR for the newly added API:
> https://bugs.openjdk.org/browse/JDK-8299680

Jan Lahoda has updated the pull request incrementally with one additional commit since the last revision:

  Changing ExecutionEnv.console() to Optional, as suggested.

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Changes:
  - all: https://git.openjdk.org/jdk/pull/11695/files
  - new: https://git.openjdk.org/jdk/pull/11695/files/d58e53f1..cc9c6f74

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=11695&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=11695&range=01-02

  Stats: 18 lines in 3 files changed: 7 ins; 0 del; 11 mod
  Patch: https://git.openjdk.org/jdk/pull/11695.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/11695/head:pull/11695

PR: https://git.openjdk.org/jdk/pull/11695