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

[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

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Commit messages:
 - Adding support for IO.readln()
 - 8342936: Enhance java.io.IO with parameter-less println()

Changes: https://git.openjdk.org/jdk/pull/21693/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=21693&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8342936
  Stats: 214 lines in 13 files changed: 197 ins; 8 del; 9 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