[wpiunits] Make Velocity.mult(Time) return Measure<D> (#7162)

Update code generator to allow arbitrary implementations of multiplication methods.

wpiunits/src/test/java/edu/wpi/first/units/measure/VelocityTest.java

@@ -0,0 +1,27 @@
+// Copyright (c) FIRST and other WPILib contributors.
+// Open Source Software; you can modify and/or share it under the terms of
+// the WPILib BSD license file in the root directory of this project.
+
+package edu.wpi.first.units.measure;
+
+import static edu.wpi.first.units.Units.Milliseconds;
+import static edu.wpi.first.units.Units.Seconds;
+import static edu.wpi.first.units.Units.Volts;
+import static org.junit.jupiter.api.Assertions.assertInstanceOf;
+
+import edu.wpi.first.units.Measure;
+import edu.wpi.first.units.VelocityUnit;
+import edu.wpi.first.units.VoltageUnit;
+import org.junit.jupiter.api.Test;
+
+class VelocityTest {
+  @Test
+  void velocityTimesTimeReturnsDivisor() {
+    var velocity = VelocityUnit.combine(Volts, Seconds).of(123);
+    var time = Milliseconds.of(100);
+    Measure<VoltageUnit> result = velocity.times(time);
+    // Compile-time test - would fail to compile if the return type was Mult or Measure<?>
+
+    assertInstanceOf(Voltage.class, result);
+  }
+}