Add module-info.java to flatbuffers-java for JPMS support

[dfa1]

Problem

flatbuffers-java ships without a module-info.java, so it becomes a filename-based
automatic module on the Java module path. This has two consequences:

1. The module name (flatbuffers.java) is derived from the JAR filename and can change if the
   artifact is renamed or shaded — making it unreliable to requires in downstream
   module-info.java files.
2. Maven Surefire (and other tools) warn that projects depending on filename-based automodules
   must not be published to Maven Central as proper modular JARs:

[WARNING] Required filename-based automodules detected: [flatbuffers-java-25.2.10.jar].
Please don't publish this project to a public artifact repository!

Expected

flatbuffers-java ships with a module-info.java that:

• declares a stable module name (e.g. com.google.flatbuffers)
• exports com.google.flatbuffers

Suggested change

Add java/src/main/java/module-info.java:

module com.google.flatbuffers {
    exports com.google.flatbuffers;
}

This is a non-breaking addition. Classpath users are unaffected; module-path users get a
stable, named module.

Environment

• flatbuffers-java 25.2.10
• Java 25 / JPMS