RFR: 7903121: initial jextract tool code, tests and samples [v2]

[Athijegannathan Sundararajan]

> Adding initial standalone jextract code, tests and samples.

Athijegannathan Sundararajan has updated the pull request incrementally with one additional commit since the last revision:

  Fixed README.md for consistent section headers

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Changes:
  - all: https://git.openjdk.java.net/jextract/pull/2/files
  - new: https://git.openjdk.java.net/jextract/pull/2/files/d3a7edfe..a9ad807c

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jextract&pr=2&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jextract&pr=2&range=00-01

  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jextract/pull/2.diff
  Fetch: git fetch https://git.openjdk.java.net/jextract pull/2/head:pull/2

PR: https://git.openjdk.java.net/jextract/pull/2