rebasing jextract on top of JDK 19

[Maurizio Cimadamore]

Hi,
as we have the first JDK 19 EA [1] build containing the changes for JEP 
424 [2], we would like start adding support for these changes to the 
jextract repo [3].

To do this, we will freeze the current jextract in a separate branch, 
called "jdk18". Developers who would like to work against 18, can 
checkout that branch and work from there.

The "master" branch will instead be updated to support the latest JDK 19 
API.

We plan, moving forward, to stick to this approach (e.g. every time a 
new JDK N + 1 is available, "master" will track those changes, and a new 
branch for JDK N will be created).

Cheers
Maurizio

[1] - https://jdk.java.net/19/
[2] - https://openjdk.java.net/jeps/424
[3] - https://github.com/openjdk/jextract