exports: add RehearsalExpression

see also #935

src/MusicalScore/VoiceData/Expressions/index.ts

@@ -8,4 +8,5 @@ export * from "./InstantaneousTempoExpression";
 export * from "./MoodExpression";
 export * from "./MultiExpression";
 export * from "./MultiTempoExpression";
+export * from "./RehearsalExpression";
 export * from "./UnknownExpression";