Introduce `org.junit.onramp` module

[sormuras]

In Paving the on-ramp Brian Goetz explains the goal and role of Java's on-ramp in great detail. Most of the topics were recently shipped in JDK 25:

• JEP 511: Module Import Declarations
• JEP 512: Compact Source Files and Instance Main Methods

void main() {
  IO.println("Hello World");
}

Let's introduce an org.junit.onramp module to allow users to write minimal HelloJUnit.java source programs:

import module org.junit.onramp;

void main() {
  JUnit.run(this);
}

@Test
void stringLength() {
  Assertions.assertEquals(11, "Hello JUnit".length());
}

Work-in-progress API definition:

module org.junit.onramp {
	requires static transitive org.apiguardian.api;
	requires static transitive org.jspecify;

	requires transitive org.junit.jupiter; // for writing tests
	requires org.junit.platform.launcher;  // for running tests

	exports org.junit.onramp; // with sole `JUnit` class
}

Deliverables

• Commit to the module name org.junit.onramp or find a better one.
• Include a minimal JUnit.java implementation and documentation.
• Update User Guide describing the purpose and usage of the new module.
• Update build to test and deploy the new module.