RFR: 8344706: Compiler Implementation of Compact Source Files and Instance Main Methods

[Jan Lahoda]

This is a PR that implements JEP: Compact Source Files and Instance Main Methods. Changes include:
- `java.io.IO` moved to `java.lang.IO`, and no longer uses `System.console()` to implement the methods (thanks to @stuart-marks)
- `java. ... .IO` is no longer automatically imported in any compilation unit
- the feature is finalized (i.e. no longer requires `--enable-preview`)

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Commit messages:
 - Moving tests as suggested, removing unused import.
 - Fixing test.
 - Fixing test.
 - Fixing test.
 - Removing unnecessary enable preview from tests.
 - Merge branch 'master' into finalize-concise-source-files
 - Merge remote-tracking branch 'stuart-marks/JavaLangIO' into finalize-concise-source-files
 - Responses to Alan's comments: clean up readln() and align with class spec.
 - Clean up some charset stuff, use stdin.encoding property.
 - Merge branch 'master' into JavaLangIO
 - ... and 16 more: https://git.openjdk.org/jdk/compare/1c2a5533...a5d223e1

Changes: https://git.openjdk.org/jdk/pull/24438/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=24438&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8344706
  Stats: 962 lines in 56 files changed: 294 ins; 585 del; 83 mod
  Patch: https://git.openjdk.org/jdk/pull/24438.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/24438/head:pull/24438

PR: https://git.openjdk.org/jdk/pull/24438