RFR: 7903788: preparation towards json configuration for jextract tool [v2]

[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.

Athijegannathan Sundararajan has updated the pull request incrementally with two additional commits since the last revision:

 - removed -Xlint:unchecked. Was a left-over from debugging
 - got rid of JWCC, tests and JSON.parse's flag for extension support.

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Changes:
  - all: https://git.openjdk.org/jextract/pull/257/files
  - new: https://git.openjdk.org/jextract/pull/257/files/9c455fdb..7cfaa5aa

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jextract&pr=257&range=01
 - incr: https://webrevs.openjdk.org/?repo=jextract&pr=257&range=00-01

  Stats: 308 lines in 5 files changed: 0 ins; 306 del; 2 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