RFR: 8342936: Enhance java.io.IO with parameter-less println() and readln() [v2]

[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:

  Removing incorrect sentence about writing a prompt for the prompt-less readln method.

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Changes:
  - all: https://git.openjdk.org/jdk/pull/21693/files
  - new: https://git.openjdk.org/jdk/pull/21693/files/3554045b..78bf15bf

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=21693&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=21693&range=00-01

  Stats: 2 lines in 1 file changed: 0 ins; 1 del; 1 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