Implement and test default string formatting in JVM and JS

Relax Duration.toString tests for JS and Native rounding specifics

libraries/stdlib/js/src/kotlin/time/formatToDecimals.kt

@@ -5,6 +5,24 @@
 
 package kotlin.time
 
-internal actual fun formatToDecimals(value: Double, decimals: Int, unitName: String): String {
-    TODO("not implemented")
+import kotlin.js.json
+import kotlin.math.*
+
+internal actual fun formatToExactDecimals(value: Double, decimals: Int): String {
+    val rounded = if (decimals == 0) {
+        value
+    } else {
+        val pow = 10.0.pow(decimals)
+        @Suppress("DEPRECATION", "DEPRECATION_ERROR")
+        kotlin.js.Math.round(abs(value) * pow) / pow * sign(value)
+    }
+    return rounded.asDynamic().toFixed(decimals).unsafeCast<String>()
+}
+
+internal actual fun formatUpToDecimals(value: Double, decimals: Int): String {
+    return value.asDynamic().toLocaleString("en-us", json("maximumFractionDigits" to decimals)).unsafeCast<String>()
+}
+
+internal actual fun formatScientific(value: Double): String {
+    return value.asDynamic().toExponential(2).unsafeCast<String>().replace("+", "")
 }