Make DurationUnit not a typealias of TimeUnit on JVM

Provide conversion functions between DurationUnit and TimeUnit

libraries/stdlib/test/time/DurationTest.kt

@@ -92,6 +92,7 @@ class DurationTest {
             repeat(10) {
                 val d1 = value.toDuration(unit)
                 val unit2 = units.random()
+                @OptIn(ExperimentalTime::class)
                 val value2 = Duration.convert(value, unit, unit2)
                 val d2 = value2.toDuration(unit2)
                 assertEquals(d1, d2, "$value $unit in $unit2")
@@ -200,6 +201,7 @@ class DurationTest {
             val unit = units.random()
             val unit2 = units.random()
 
+            @OptIn(ExperimentalTime::class)
             assertAlmostEquals(Duration.convert(value.toDouble(), unit, unit2), value.toDuration(unit).toDouble(unit2))
         }
 

libraries/stdlib/test/time/DurationUnitTest.kt

@@ -19,6 +19,7 @@ import kotlin.time.DurationUnit.NANOSECONDS
 class DurationUnitTest {
 
     @Test
+    @OptIn(ExperimentalTime::class)
     fun conversion() {
         fun test(sourceValue: Double, sourceUnit: DurationUnit, targetValue: Double, targetUnit: DurationUnit) {
             assertEquals(