Make nashorn an optional (static) module dependency

nashorn is used only by beastlabs.util.Script and was removed from the
JDK. As a hard 'requires' it made the whole beast.labs module
unresolvable wherever nashorn is absent, so the package manager skipped
the module and every BEASTLabs class became unloadable. BEAUti then
crashed on startup parsing the Weibull subtemplate (ClassNotFoundException
for beastlabs.math.distributions.WeibullDistribution). Declaring it
'requires static' lets the module resolve without nashorn, matching how
beast.fx and javafx.controls are already handled.

Fixes #30

Author: alexeid

src/main/java/module-info.java

@@ -4,7 +4,10 @@
     requires java.xml;
     requires java.scripting;
     requires beagle;
-    requires org.openjdk.nashorn;
+    // Optional: only beastlabs.util.Script uses nashorn, which was removed from
+    // the JDK. A hard 'requires' makes the whole module unresolvable (and thus
+    // skipped) wherever nashorn is absent; 'static' keeps it optional at runtime.
+    requires static org.openjdk.nashorn;
     requires org.apache.commons.statistics.distribution;
     requires org.apache.commons.numbers.gamma;
     requires static beast.fx;