jextract standalone repository

[Maurizio Cimadamore]

Hi,
as anticipated in [1], jextract has finally landed in its own standalone 
repository:

https://github.com/openjdk/jextract

The version of jextract included in this repo is suitable to work with 
Java 18 (just hot off the press!), and we plan to create new branches as 
new Java versions will come out (to make it easy to find the version you 
want to work with).

The jextract sources can be built using gradle; as usual, the build 
depends on libclang, so a LLVM binary snapshot is required [2]. Testing 
is also possible (requires jtreg).

Jextract binary snapshots will be made available at a later date. Moving 
forward, we will gradually phase out the jextract branch 
(foreign-jextract) of the panama/foreign repository, and work on the 
standalone repository instead.

If you are interested, please give it a try, and let us know what you think.

Cheers
Maurizio

[1] - 
https://mail.openjdk.java.net/pipermail/panama-dev/2021-December/015895.html
[2] - https://releases.llvm.org/download.html