Java (OpenJDK) using commonmark-java

Since this pull request, Java (OpenJDK) has included a copy of commonmark-java:

• https://github.com/openjdk/jdk/pull/16388

When they copied the source files, they made a few adjustments too. There's a list here:

// This module is primarily an import from a recent tagged version of
//     https://github.com/commonmark/commonmark-java
//
// The following parts are imported:
//
// * commonmark/src/main/java
// * commonmark/src/main/resources
// * commonmark-ext-gfm-tables/src/main/java
// * commonmark-ext-gfm-tables/src/main/resources
//
// For source and resource files, the following transformations are applied:
//
// * legal headers are added
// * package and import statements are updated
// * characters outside the ASCII range are converted to Unicode escapes
// * @SuppressWarnings("fallthrough") is added to getSetextHeadingLevel
// * the value for ENTITY_PATH is updated with the modified package
// * the file `entities.properties` is renamed to `entities.txt`

Let's make it easier for them next time by adopting some of the changes.

Beginne damit, die Pfade commonmark/src/main/java und commonmark/src/main/resources sowie commonmark-ext-gfm-tables/src/main/java und src/main/resources mit den kopierten OpenJDK-Dateien zu vergleichen, auf die in module-info.java verwiesen wird. Prüfe jede aufgeführte Transformation sowie die zugehörige Anordnung von Quellcode und Ressourcen. Erledigt ist die Aufgabe, wenn die relevanten Unterschiede behoben sind, sodass ein künftiger OpenJDK-Import weniger Anpassungen erfordert.