RFR: 8342936: Enhance java.io.IO with parameter-less println() and readln() [v3]

[Jan Lahoda]

> This PR is simply adding parameter-less `java.io.IO.{println(),readln()}`, with the (hopefully) obvious semantics, plus the corresponding wiring to make those work. This may become part of JEP 495:
> https://openjdk.org/jeps/495

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

  Fixing test for Console.readln/readLine, ensuring the proper methods are called.

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Changes:
  - all: https://git.openjdk.org/jdk/pull/21693/files
  - new: https://git.openjdk.org/jdk/pull/21693/files/78bf15bf..eadbc685

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=21693&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=21693&range=01-02

  Stats: 46 lines in 3 files changed: 29 ins; 13 del; 4 mod
  Patch: https://git.openjdk.org/jdk/pull/21693.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/21693/head:pull/21693

PR: https://git.openjdk.org/jdk/pull/21693