Prepare code, tests and docs for Java 18

[kriegaex]

Core:

• JDT Core dependency in pom.xml
• Constants.java
• LangUtil.java
• AjcTask.java
• messages_aspectj.properties

Test:

• XMLBasedAjcTestCaseForJava17Only.java
• XMLBasedAjcTestCaseForJava18*.java
• tests/bugs199
• tests/features199
• refresh AspectJ libraries under lib/aspectj and lib/test

Doc:

• JavaVersionCompatibility.md
• README-199.html

CI:

• GitHub CI build

Closes #144.

[kriegaex]

Status: Java 18 tests for 2nd review of pattern matching for switch are not working yet, the upstream implementation in JDT Core does not seem to be finished yet. Therefore, those tests are inactive at the moment.

[kriegaex]

Status quo:

• All active tests are green.

• The tests specific to preview 2 of switch patterns are still inactive because of unsolved upstream bugs

  • 576746,
  • 579355,
  • 579360 (scheduled for 4.24).

  I want to wait for the first two to be merged, then refresh our fork. The last one should not block the 1.9.9 release, I want to publish it soon.

• In order to get the tests green again, I had to switch several LTW tests from embedded (in-process) to full LTW mode, i.e. they run in a separate process with the weaving agent on the command line. This is because under Java 18, those in-process tests need code paths using restricted access to JRE classes, i.e. we need --add-opens on the command line, just as described in the 1.9.7 release notes for LTW. I could have added that option to the test runner for Java 16+, but wanted to avoid it, because we would become blind for situations, in which non-LTW tests would run into similar problems in the future. So I rather changed a couple dozen LTW test configs. In doing so, I also had to adjust other things, because suddenly we have tests requiring passed-through main method arguments in full LTW mode, we have more debug output about unweavable AspectJ core classes in some tests etc.
• The biggest issue I fixed was that full LTW tests were not finding the java agent at all, but none of them failed despite error messages on the console, because they do not assert on the log output in any way. Only one test which I had to switch from embedded to stand-alone actually checked something (a class file dumped by the weaver), making the test fail. When inspecting the log output, I noticed that much more was wrong there. So I committed an up-to-date aspectjweaver.jar to the libs directory and pointed the path in the test runner to there. I also refreshed other AspectJ libraries in libs, while I was at it anyway.
• I still do not like the binaries in libs after I got rid of most of them already. But in order to not need them anymore, we have to reorganise the tests depending on them (such as LTW tests) to be integration tests. For instance, the LTW tests should go to the aspectjweaver module and be run in the verify phase. That would however impact the special Eclipse "run them all before commit" configuration you (@aclement) like to use. Apart from that, it would just be a whole bunch of work I was not willing to do just for the Java 18 release.