RFR: 7903788: preparation towards json configuration for jextract tool

[Athijegannathan Sundararajan]

* adding json skara code into org.openjdk.jextract.json.parser package. Minor changes done - pretty printing, JSONValue is made sealed and access changes (private final fields).
* org.openjdk.jextract.json package for JSON interface to the rest of the jextract tool. Main interface is org.openjdk.jextract.json.JSON. parse and toString methods to convert record into JSON String and vice versa.
* Other classes are public mainly for testing purposes. build.gradle adjusted with the required --add-exports clauses so that jextract module does not have to export the new json packages.

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Commit messages:
 - 7903788: preparation towards json configuration for jextract tool

Changes: https://git.openjdk.org/jextract/pull/257/files
  Webrev: https://webrevs.openjdk.org/?repo=jextract&pr=257&range=00
  Issue: https://bugs.openjdk.org/browse/CODETOOLS-7903788
  Stats: 5132 lines in 20 files changed: 5132 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jextract/pull/257.diff
  Fetch: git fetch https://git.openjdk.org/jextract.git pull/257/head:pull/257

PR: https://git.openjdk.org/jextract/pull/257